Everybody's suggestions for training them to walk on when you want them to are good............ it's no different for any animal.
You don't want to be PULLING her, ever. You want her to give in to your command to walk on.
Same with a colt (or a child). They need to understand and recognize who is in charge, and what their role in relationship to that hierarchy is. THAT'S why you tie them to the post, and walk away. The post is NOT going to pull on her, but it also is not going to give in to her... no matter how hard she might pull. It WILL however establish dominance, period, and she WON'T win against it, unless the post or the rope breaks. In that case, you need bigger of both (thus the tractor instead of a Ranger which she might be able to pull against and win).
However, as soon as she does "give" to the post, she is automatically and immediately rewarded with a release of the pressure... EVERY time, period. She'll learn pretty quickly not to pull and to "release" her pressure by moving forward, as soon as she feels it applied on the halter. Then you will use that learned response to your advantage by rewarding her when she "gives to you" when you're on the end of the lead rope instead of the post. If she gives you one step, DON'T immediately ask for another in sequence (asking for more than she's ready to give you), but instead, reward her, tell her she's a good girl, scratch her on the head, stroke her neck. THEN ask her to give you that ONE STEP again. DON'T fight with her, and for sure, DON'T PULL HER WITH THE TRACTOR...... the tractor is just so that your "moveable post" is big enough for her not to be able to establish her dominance over it by dragging it in the direction SHE wants to go. YOU always have to be the dominant one in the relationship. But YOU also have to be a kind, pleasant individual that she appreciates being around, and that TEACHES her that she did the right thing by giving in to your applied pressure. Dragging her around a pasture surely isn't going to help in the least.
It's all about asking for a response, her recognizing and understanding the ask, and then you rewarding her for giving you that response. Be consistent in your asking... always using the same cues. And ALWAYS give her that reward when she responds appropriately. Eventually, you'll be able to ask for more ( ask for two steps in a row, then four... then ten,... then walking on and her just following behind you without ever even tightening up the lead rope), and she'll be willing to give you more. Done right, she'll "learn" fairly quickly that this is a "learning trajectory"......... that always uses the same formula.............. asked for response, proper response given, reward given. And remember, the preferred ultimate "reward" is the release of the "pressure". That's how you get to those beautiful "drooping loose reins" on a reining horse, for example, or a hanging relaxed head on a really good cutting horse. Watch any of these on a really well trained animal, and you can hardly see the rider "cueing" the animal at all............ that's because the cues have become so subtle they're almost imperceptible. THAT'S what the judges WANT to see in the showing with your heifer or steer too.... a well trained animal that responds appropriately to its "master".
Don't "ask" for more than she's ready to be able to give to you.... they need to walk before they know how to run.