You may have different reasons to halter break your cows and teach them to lead. Maybe your child wants to show them in livestock shows; maybe you want easier loading and unloading for trips to the vet; maybe you just want pets of your cows; or maybe you plan to milk your cow.
The hardest part of getting started on halter breaking a cow is getting the halter on the calf the very first time. This can be done over a long period of time or a short period of time. If you have plenty of time, you can accomplish this task by penning the calf for a portion of the day (few hours) away from its mother. Over time, gentle the calf by feeding it calf starter feed. Each day get closer to the calf until finally it allows you to put your hands on him while he eats. Never do anything that frightens your calf and he will quickly learn that your voice, your presence, and finally your touch will not hurt him. Over time you will be able to put that halter on his head without a fuss -- but this is a steady and patient task. Be sure to fasten the halter snuggly.
If you don't have the time to go through the process above, you can choose a speedy route but one that is forced upon the calf. In the case you will need either a restraint set up at your farm or you will need to transport your calf to the vet who has a restraint. If you need quicker results, you will need to restrain the calf to get the halter on the calf for the first time. Be sure to clip on a lead rope while the calf is restrained.
After the halter is on the calf, you need to teach the calf that he can't win in the fight with the rope. This is important because eventually the cow may weight 10 times your weight and could drag you at will. This is best done by securring the lead rope to a stationary firmly placed object - like a tree or a secure fence post. Be sure to tie the rope in a quick release fashion - we call it a stable knot - just in case you get a problem and need to untie the calf quickly. Tie your calf up every day for an hour or so. At first he will protest but over time he will learn that the rope will not let him go. Be sure to leave enough slack in the rope that the calf can stand up and lie down, but not enough that he can get his feet tangled.
After he has a firm respect for the rope, as judged by his stopping fighting it and pulling on it, begin to attempt to lead him. This won't go well at first, but keep trying. Lead him to something he likes, like calf starter food, hay, or water. Wear gloves so you can hold the rope firmly without getting rope burns. This is not a task for a small child in my opinion just because they won't have the strength nor body weight to hold the calf the first few times.
Over time, continuing to tie up the calf for an hour or so a day, leading it to food, it will begin to cooperate well with the halter and the lead rope. You will really have a fun time with your cow now being able to lead it where you want it to go. Your cow will begin to enjoy you more too - understanding that time spent with you leads to food, water, maybe brushing or scratching, maybe leading to patches of green grass in your yard.
Be Patient - this is a process that will take time but is well worth it. Never frighten your calf during this process or things will not go well. Remember you're building trust and respect, not fear with your animal.
Try to never let the calf get away from you. If this happens, it really set back the process a lot because the calf learns certain tricks to use and loses respect for the rope.