Start with a Leash Exploration
If your dog is not leash trained, you may want to work on this first before moving your pet indoors. Once your dog understands that on the leash, you are in control, you can use that power to work your way inside. Start by bringing the dog into the house while on a leash so he can check the place out without running wild. If the dog behaves well, reward him with a treat. If not, take the necessary disciplinary actions and try again next time.
Create an Enclosed Environment
When you decide to let your dog off the leash in the house, keep him in a confined area. This may be a garage, a den, or a bedroom that you can seal off with baby gates and doors. Set up a place for your dog to eat, sleep, and play in this confinement, and put newspapers or puppy pads on the floor to make accidents an easy cleanup. Accidents are common when you first transition a dog inside because he is used to going when he wants, where he wants. You will have to train him to think otherwise.
Be Visible and Interactive
You want your dog to feel comfortable in the new space you have created for him, not trapped in a cage. Don't just leave your dog on his own to explore. Be present with him in the room, or make yourself visible from the gates or doorways. If you notice the dog behaving well, you can once again reward him with treats or toys. If he becomes overly agitated, you may need to let him outside for a while and limit the amount of time indoors for now.
Extend the Length of Time Indoors and Available Play Area
As your dog gets more familiar with indoor life, you can start to let him into more areas of the house for longer periods of time. You need to be able to trust your dog not to use the potty in the house and not to chew on your expensive furniture. Accidents will happen, but that is why you must make sure he is well acclimated to the indoors before letting him wreak havoc on your home. Once you feel comfortable with your dog and his new life, you can let him stay in full time.
Note that it may take a while for your dog to get used to using the bathroom at certain times, eating at certain times, etc., particularly in the night hours. Be patient through this process, and you will soon have an amazing companion to enjoy in your life.