I think if you are following online instructions, you'll do fine...most of the one's I've seen are pretty good. If you want my mom's exact instructions when she taught ME it was "Worm your finger in and if you feel a hard bump, try and get your finger behind it and roll it out. Be gentle or he'll kick you in the head." Some things you just have to learn through trial and error:P
If he doesn't want to extend, don't worry about it, just do the best you can with what you've got. Even a partial cleaning is more than horses in the wild get. If you're really worried about it, just ask your vet to do it once in awhile.