Sit quietly and wait for her to approach you. when she does, don't pet her,
toss a treat away from you behind her (as long as it doesn't scare her).
It's like a double treat. Relief from pressure and food.
Later, when she is enjoying the game and she solicits attention from you,
pet her for 5 seconds and stop. If she asks for more pets (by nudging or
pawing) then do another very short pet. The work your way up.
For more details google
"dose your dog really want to be petted" and
"youtube kikopup handling shyness"