Put her in front of a fence (or beside, whatever you need) and mount her there.
Spend a lot of time mounting and dismounting. Don't go for a ride. Once you are on, don't let her move. If she does, don't get off until she has stood still for you. You can start with a short time period to stand still and extend it until she gets the idea.
This is an exercise on it's own. Also, provide a treat or praise only when she is still, not before. Soon enough she'll figure it out and stand still for that reward.