an easy way is by using your thumb and index finger in the shape of a check mark, and run it down it's spine. when u reach their bottom , push down easily, and since it's a spot which enables you to make them sit easy, they should sit. and when they do, say "Sit!"

then if they understand, use treats for motivation. ask them to sit and if they do, say GOOD DOGGIE!!!!!! and praise them a LOT...and give them treats right after they do it.