Ok, you've done a great thing: you have rescued a dog. You have brought the adult dog home and realized that your dog needs help in the potty department. Good news: you CAN teach your old dog this new trick, sometimes in as little as three days.
(1) Take time off work (or hire a pet sitter) to train your dog. Someone has to be there to take your dog out for bathroom breaks several times a day.
(2) Use a crate from the first time you bring the dog into your home. Dogs don't like to mess up their sleeping areas. The crate should be just big enough for your dog to stand up, turn around, and lie down.
(3) Take your dog out to potty at least six times a day. Never keep your dog in the crate longer than they can hold it. Take your dog out to the same spot in your yard each time. Use a phrase such as "Go potty" or "Do your business." Before long the command will become associated with the activity and your dog will eliminate on cue.
(4) Give abundant praise when he gets it right. Give a treat immediately for successes. Positive reinforcement is the key!
(5) Clean up accidents with an enzyme cleaning product that gets rid of the odor. Don't punish your pet for an accident that you did not see occur. If you see your dog eliminating in your house, shout or clap your hands and quickly take him outside. Punishment for mistakes actually makes house training harder.