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Before your puppy or adult dog learns to eliminate in his litter box, he'll need to be able to enter it on his own. You need to teach him that it is a safe, even fun space. Praise the dog for being in the box. Repeat your command, and again, praise your dog and act happy that he is in the box. Keep practicing until you are able to guide your dog into the box while giving the "use the box" command. Once your dog is comfortable being guided into the box, try using only your verbal command. Be patient and don't repeat yourself. If your dog does not get in, simply walk away and try again later, or go back to guiding. If your dog does get in on command, praise him profusely. Keep practicing until he gets in immediately on command.
Teach your dog to go into the box on command first. Place your dog in the box and give a command like "use the box". Wait for your dog to leave, and put him back in. Have your dog enter the box with the "use the box" command alone.