Get you cat(s) and treats in the same spot so that you have the treats near by.
Grab some treats and put them in your hand.
Hold it in your hand so that the cat(s) can sniff the treats but not eat them.
Hold one treat in your hand so that the cat(s) can see it there.
Say "sit", walk towards them, make yourself look big and shadow over your cat.
Praise him/her and give them the treat.
If they don't sit when doing the previous step, continue saying "sit" and gently push their bottom down so they know what has to be done and what "sit" means.
Practice this daily so that your cat has a better understanding of what "sit" means, and what has to be done.
Remember the more practice they get, the better they become at s-it...
... Soon enough you might be able to do it without treats!
Thanks for watching!! Good luck when training your cat!!