Me and my family used to foster all sorts of cats and many times we'd have cats that didn't go to the bathroom in the litterbox so we invested in a litter box where the cat crawls into and does its business then you simply pop off the top of it later and clean. Also we found that it helped to get a litter box that is low to the ground such as one of those tin cooking pans filled with litter. Good Luck!
My torby calico, Nikki has a bad habit of pooing beside the litter box instead of in it. Nikki's problem is that she is used to going outside and doing her business. She was a completely outdoor stray cat that I started feeding and eventually took with me when I moved in with my husband. She's now an indoor/outdoor cat. As long as we let her out when she meows she doesn't make any mess.
Other than that all of my other cats litter train themselves. I agree with the person that says get a litter box with short sides. Also, when it poos outside of the litter box, pick up the poop and put it into the litter box while the kitten is watching. That might help.