Same as any other animal - pressure and release.
Get them used to the feeling of the halter on their face first; if they like treats, treating them while they wear it without its being connected to anything.
Then clip on a lead rope and gently but firmly apply pressure towards yourself. The instant they move into the pressure, even just relaxing their neck into the pressure, release the pressure. (That's their reward - giving in to the pressure releases the pressure.) Repeat. Repeat repeat repeat. After about 25 repeats over a couple of sessions, very gradually increase the movement you expect before they get the reward.
If they pull back, maintain the pressure but do not increase it. You don't want them frightened or hurt or upset, but they need to know that the only way to release the pressure is to move into it.
The above is why this approach is different to tying them to a fence. The fence can't go with them to maintain but not increase the pressure, so if they pull back from a fence it gets more and more painful and they get more and more upset. They often do learn that the only way to release the pressure is to give into it, yes, (or they may just hurt themselves and/or fling themselves on the ground), but they also learn that the halter is to be feared and avoided.
If they start kicking up / getting upset, go back a stage. Finish each session on a high, even if it's a back to stage 1 high.
Try it. You will be amazed.