Just because your new dog is old enough that he should know where to potty doesn’t mean that he actually does. An adult dog has the necessary muscle control to hold it for longer periods of time, but if no one ever taught him the potty training rules then he’s the equivalent of a brand new puppy. Age should not be equated with housetraining prowess, so it’s a good idea to treat any dog new to your home as if he’s still learning the potty training ropes.
Understand your puppy's particular breed behavior and needs. Research your dogs breed traits and special needs or any behavior that you should be aware of and look out for. For example, If your puppy is a tiny little chihuahua, their bladder will be very small and they will need to urinate more frequently; accidents will happen even if they are well trained.
There are a lot of different ways to train your dog. You may choose to sign up for a dog training class, hire a professional dog trainer for private lessons, or even send your dog to board with a trainer. However, plenty of people successfully train their dogs on their own. It's a great way to save money on training costs, and a wonderful way to bond with your dog.
The process is the exactly same for older dogs as it is for puppies; prevent accidents from happening by using baby gates and a crate if possible, create a predictable schedule, learn to recognize your dog’s potty signals, accompany your dog outside and immediately reward for all outdoor elimination, properly clean any accidents and supervise your dog until you’re sure that he understands the rules.