Ok well you put the lead rope through her front legs, and lift her front left leg up and SOFTLEY tug on the lead rope (and that will pull her head under). And very very softley pull her leg that you are holding back like a millemetre. And keep on doing that and she will eventually figure out what you are asking from her.
Don't FORGET !!!!! - if she leans back even a millemetre, STOP and give her a pat and then that will do for the day. Then the next day do it again, you need to make sure that you don't do it to much or she will get sick of it and wont do it for you. So like only do it about 3 times a day even if she don't lean back, she will eventually get it so just keep on trying it over weeks and weeks and she will get it