Always tell them to do 3 things. Any more than that and they get 'lost' trying to think too much.
#1. Set up in a cruched ball position with arms straight...I don't think 'locked' is a good idea...let the boat pull you, don't try to pull back.
#2. As the boat starts to pull keep having them repeat this to themselves..."Pull my heels to my butt, pull my heels to my butt". The biggest mistake beginners make is thinking they are trying to get the board 'on top' of the water. That's not true, they should be trying to pull it 'under' the water and let Bernoulli's principle work for them.
#3. Once they are up, it's not a race to stand...stay crouched and rotate the handle to your lead hip (right for goofy, left for regular).
I just keep saying, "Bring your heels to your butt" It works for everyone who is under 200 lbs and strong enough to hold on to the rope. For big guys...well, get a bigger board than a 139