First thing in the morning, its easy because they've been holding it all night. But they do tend to think of it like a game at first. Just be patient and persistent with them. When he steps out of the box we pick him up and repeat "go potty elvis" everytime, then we put him back in until he finally does go. There will be days when it seems like they just don't get it, but they will. If they make a mistake we try to catch them in the act and quickly put them in the litterbox and tell them to go potty. If they finish their business there, we praise them again. Most of the time, its a matter of keeping them in the confined area of the kitchen and watching them. When they act like they need to go, we put them in the box and tell them to go potty. I know it may seem like a hassle, but any kind of indoor training is tough at first.
Now Elvis has full reign of the house, we just keep a litter box on every floor of the house and he knows where they are and uses them 100% of the time. When we take him for walks or he plays outside, he has no problems going outside and still using his indoor litterbox when he returns to the house. It is also great because most of the time he is using the litterbox, so my yard remains lush and green, where our neighbors yards have dead spots from repeated dog urination. We also have a female yorkie who is 17 weeks now and she has caught on pretty quickly. I think it helped to already have a yorkie that uses it. Hang in there. Just be calm, patient, and persistent and ALWAYs try to keep the litterbox as clean as you can. Your baby will catch on! Best of luck!