Here's how I train pups to hold their leg still for me. It takes a while, maybe 3-4 sessions before they get it (and I have to add that some NEVER get it). It takes time and patience but will pay off in the long run.
First, I pick up the foot and hold gently for a couple seconds, then set it back down.
Gradually, I increase the time that I hold the foot, without doing anything to it. If the pup starts to struggle and pull the leg back, I hold just a little bit more firmly. No scolding or talking to the pup at this point. Eventually, the pup will stop struggling, at least for a moment to catch his breath, and at the moment the leg relaxes, I let go. Soon I will increase the amount of time the pup's foot must stay relaxed before letting go; I try to count to three.
This teaches the pup that struggling only makes me hold the foot more firmly, and that relaxing makes me let go. Once this is established, I can start clipping nails, scissoring the foot or whatever I need to do. Of course this will probably cause the pup to struggle again, but I just repeat the process. Hold firm until the pulling stops, count to three, then let go. Yes, it can be tedious and time-consuming, but we're aiming for the long run here.
After a few sessions, the pup should understand what is expected of him. If he continues being bratty into the 4th-5th session (he will probably be an adolescent or adult by then) I will discourage pulling by snapping my teeth. It's amazing how this simple act will put most dogs into a submissive state. You just get your face close to his face, show all your teeth and click them together. You don't need to make a sound, the pup understands what this means--"You'd better behave!" Usually it is followed by face-licking and an "I'm sorry" attitude. Praise of course when the pup holds his leg still.
For those dogs that just NEVER get it, pushing the leg backward when he pulls backward will activate the opposition reflex and cause him to move his foot forward.