Method 1: Targeting.
You will need an object that is the size of your dogs paw and is kind of flat. Teach them that if they teach the target to get a treat. put the target on the other side.
step 1 : Luring.
Lure them down then lure them to reach with the other paw if this does not work, try the targing object. If they try to stand up and use the wrong paw, put them back in the down position.