Start with super low cross railes in walk trot then if they feel ready canter then go a little higher if they are ready
start low, just with poles walking, then trot etc. until she is comfortable with that start working up. If she is nervy let her go up and smell the jump before you attempt it.
Small jump. And try to not do anything but steer her towards the jump. Sit in the two point seat so you won't bother her. Do this a lot and she will build confidence and figure out how to jump how she feels best
Let the horse walk over the jump first if the horse can