That's really the basics of it. Cats naturally like to bury their excretions, so some take right to it in the right place. Others, however, are confused because there's nothing like where they used to go to the bathroom for them to use.
The potting soil (NOT outside dirt) that has no additives is the best thing to use. Put a layer over the litter. When kitty is using it reliably, scoop one of his pees into a new box that has just the litter. That is often all that's required. Caliprincess had a more difficult case, and they way she handled it was perfect. You would need a crate that is large enough for a litter box, a place to sleep, and food and water.
Our Lazlo slept on the soil. He refused to go to the bathroom for over 24 hours. We got so worried we called the vet. He'd just seen the vet though, so we knew it wasn't a blockage. Lazlo finally gave in, and it is was a whopper of a pee from such a small cat, lol! Then he had the good sense to stop sleeping on the soil.
You do need to know that if kitty ever poops or pees outside of the box, the area needs to be cleaned with an enzyme cleaner. If it's a hard surface, you can use chlorine (make sure you leave no residue behind!). Do NOT use ammonia to clean cat pee or poop. An enzyme cleaner has to be used on fabrics, and the directions must be followed. The important thing is to COMPLETELY remove the smell (from kitty's perspective, not a human nose perspective) - otherwise they will think they should still go there.
Hope you don't ever need that advice. However, when cats get infections related to their bladders or urinary tracts, they often pee outside the box. That's really important to remember - that once kitty IS litterbox trained, if he goes outside the box, in 85% of the cases it is due to a medical problem, and getting kitty to a vet to rule that out is the first thing that should be done.
Thank you for rescuing this kitty!
I hope the other kittens find homes too. And it is best if you get your kitty spayed or neutered for many, many reasons. Most vets recommend this be done at around 4 months old, though they can be spayed or neutered as young as 7 weeks without any adverse effects.
Also, you may want to think about talking to your friend about trying to trap the other kitties if necessary. Especially if they're going to remain outdoor cats that no one takes care of, it would be healther for them and the neighborhood if they were not able to reproduce in order to create more homeless cats. Something to think about - and something we can help you with if you have any questions about how to do that, or if you need to find a low-cost (or potentially free) spay/neuter clinic.