This is a little random, but about a decade ago a dear friend acquired an adult dog who wasn't housebroken, and they were at a complete loss for how to handle it.

At the time I also didn't know anything about dogs, so I did some research and compiled the useful stuff I'd been able to find. I don't know how much innovation has happened in this part of the world over the years, or how many people will ever need this particular advice, but I think some things (like how dogs relate to crates) has been useful for me in other dog situations, so I thought it might be worth sharing.

Housebreaking an Adult Dog

Sometimes an older dog who has been through a lot of change will have a housebreaking relapse. Or there are some instances where an older dog has never been housebroken. Sadly, the prevailing reason for dogs ending up abandoned or at shelters is the housebreaking issue. The dog who soils his owners home or his own crate coupled with the owner's assumption that the dog is a vengeful beast who is doing so out of spite is the principal reason millions of dogs die each year in shelters across the nation. Regardless of the issue, if your adult dog needs to be housetrained (or retrained) you need to remember that you and your new dog need some time to learn each other’s signals and routines. For this purpose, your new dog is no different than a puppy. He needs to learn how your household works and where he is expected to eliminate and where he should not do so. Fortunately, if you follow the steps below, it is not that difficult to teach an old dog a new trick! For the first few weeks after you bring your new dog home, you should assume that he isn’t housetrained and start from scratch. Just be patient and expect him to make mistakes along the way.

Establish a Routine

Generally, dogs go to the bathroom after they wake up in the morning, about 30 minutes to an hour after they eat and right before bed. These are the main times to keep in mind to start housebreaking your dog. (Typically you’ll want to give your dog at least six bathroom breaks daily until he’s housebroken—if you can’t take time off work when you first get your dog, it might be good to hire a dog-walker)

Establish a routine so that during the key times mentioned above your dog knows when he is expected to go. For example, first thing in the morning when he wakes up, when you arrive home from work, and before you go to bed. (Take him outside during these set times, whether he needs to eliminate or not). Dogs are creatures of habit; the more quickly you turn a good behavior into a habit, the faster your training will go.
Choose a location not too far from the door to be the bathroom spot. Always take your dog, on leash, directly to the “elimination station.” The smell of his urine and feces will tell him that this spot is acceptable and encourage him to go (If you clean up an accident in the house, leave the soiled rags or paper towels in the bathroom spot outside to help the dog recognize the area as the place where he’s supposed to eliminate).
Choose a word or phrase that you will use each time you are prompting him to do his business. For example, you could say "go potty" or something else that you are comfortable with. Repeat the word or phrase until he goes so that he associates it with using the bathroom. Some people recommend that you start out saying this phrase while your dog is eliminating and then you can eventually move to using before he does his business to remind him of what he’s supposed to be doing. Don’t distract your dog with games and chit-chat; just stand still and let him circle and sniff.
Praise your dog each time he goes outdoors while you are training him. In the early stages of housebreaking, give him a top-notch treat each time he relieves himself outside. You want to make it crystal clear that eliminating outside is a great thing. Once the dog is beginning to learn, simply praising with words will be enough. Note: you must praise him and give him a treat immediately after he’s finished and not wait until after he comes back inside the house. This step is vital, because otherwise he won’t connect the reward with what prompted it.
Also: take your dog for a walk or give him some playtime as a bonus reward. If he always comes straight back inside after doing his business, he’ll learn to hold it to prolong his time outdoors.
Establish a feeding schedule. Dogs only need to be fed once or twice a day--usually in the morning and afternoon. Set out a bowl of food for about 15 or 20 minutes during these times and let the pet eat. If your dog does not eat after this time, do not feel guilty about picking it up. This will help your dog learn a feeding schedule, which in turn will regulate the bathroom times. Always leave water out, though. An important fact about feeding your dog is not to change his brand of food abruptly or give too many treats or table food. Doing so may cause diarrhea.

Supervise, Supervise, Supervise

Don’t give your dog an opportunity to soil in the house. He should be supervised at all times when he’s indoors. (He should also be supervised during potty breaks so that you are able to reinforce the appropriate behavior as well as ensure that he is actually doing his business)

Watch your dog when he is inside, and keep him as near as possible (you can tether him to you with a six-foot leash or use baby gates to keep him in the room where you are, but at the very least, close all bedroom, bathroom, and any other doors to limit access around the house). This supervision will allow you to watch for any signs that he might need to relieve himself. For example, sniffing around, circling, tail arching, whining, standing at the door or staring intently at you could be signals that it is time to go outside. At the first sign, immediately take him out, on a leash, to his bathroom spot. (to succeed at housebreaking, everything that looks like a “sign” should be taken seriously (especially at first)—even if that means you’re taking your dog outside 20 times a day : ) Stay for at least 10 minutes and repeat your key phrase a few times. If he goes, praise him lavishly and give him a treat. If you do not respond to the signs, the dog will simply do his business inside. Adult dogs do not need to go as often as puppies, so you should have some time between potty breaks.

Any time your dog does not go after 10 or 15 minutes of being taken out, bring him back in. Take the dog back out about 20 minutes later.

Confinement

He should be confined to an area small enough that he won’t want to eliminate there whenever you are not home or when you’re unable to watch him (including during the night). The most effective way to teach him to eliminate outdoors is to prevent him from using the house in the first place. It should be just big enough for him to comfortably stand, lie down and turn around in, but no bigger. If it’s too spacious, your dog may feel like he can eliminate in one corner and still keep his living space clean. Dogs are den animals, so they generally will not use the bathroom inside their dens unless it is a last resort. This could be a dog crate (highly recommended), a portion of a bathroom or laundry room blocked off with boxes or baby gates. If he has spent several hours in confinement, when you let him out, take him directly to his bathroom spot and praise him when he eliminates.

Confinement will teach him to signal when he has to relieve himself and to control his bowels sufficiently until an opportunity to relieve himself is offered.

Obviously, never confine your dog for longer than he can hold it. If he’s ever forced to go inside his crate because you didn’t let him out in time, you’ve made housetraining much, much harder.

Oops!

Most dogs, at some point, will have an accident in the house. You should expect this, as it’s a normal part of his adjustment to his new home.

If you catch your dog in the act of eliminating in the house, do something to interrupt him like making a startling noise (say “no” or clap, but don’t scare him). Pick him up or do something else that makes him stop. (Dogs usually stop going to the bathroom when startled.) Immediately take him to his bathroom spot, praise him, and give him a treat if he finishes eliminating there.

Don’t punish him for eliminating in the house. If you find a soiled area, it’s too late to administer a correction. Do nothing but clean it up. Rubbing your dog's nose in it, taking him to the spot and scolding him, or any other type of punishment, will only make him confused, fearful of you or afraid to eliminate in your presence. (it will also create more stress for your dog). Animals don’t understand punishment after the fact, even if it’s only seconds later. Punishment will do more harm than good. It’s best to put the dog outdoors or in another room while you clean.

Cleaning the soiled area is very important because dogs are highly motivated to continue soiling in areas that smell like urine or feces. Use a cleaning product that contains live bacteria or enzymes that break down the mess, rather than masking it with another fragrance. Stay away from ammonia-based cleaners; they’ll smell like urine to your dog and he’ll want to pee again on the same spot.

Repeat the above process daily and be consistent with it.

One thing’s for sure: if you are not consistent with your dog, he will go back to his old habits right away. You must continue the housebreaking schedule way past the point of success and continue with the schedule — for the benefit of the dog and of your home.

And remember, just because your dog is housebroken does not mean that he can hold it for an unreasonable amount of time. In addition to regular bathroom breaks throughout the day, dogs also need a long walk every day to burn off excess energy. Why? Because pent up energy can also cause a dog to eliminate in the house out of nervousness and boredom.

Housetraining your older dog requires patience, humor, understanding, compassion and time. He wants to please you by doing the right thing. Help him make the adjustment to his new home a successful one.

Sources:

I don't have a record of all the sources I originally consulted because I was just sending this to my friend, but from googling, I identified this one: https://dogs.thefuntimesguide.com/housebreaking_older_dog/ but there were probably many.