my Chihuahua used to do that. The way I trained him was to lock him in a small room in which I covered the entire room over with piddle pads. When I was to return I praised him and gave him a treat for going poop on the pad. Then the next day I took one piddle pad off the floor and came back to see he pooped on a piddle pad and praised him again. I kept taking away one pad at a time until there was only one left and then seeing he was a chihuahua I put the pad into a small doggie litter box and trained him to go into there. It took me about two weeks for this process to work. If and when he made a mistake to go on the ground and not on the piddle pad I would not treat him and add a piddle pad to the ground where he went.

good luck