It is more difficult when the mothers aren't friendly........
I start mine right away being handled. But my cows are very friendly and most are halter broke themselves.
While small babies-
If you can put cow and calf in a corral and distract the cow with some hay and/or grain while you work, it's best.
If mom is a problem, slip her into an adjoining pen. (Mine would get tied up.)
At first you just want to get the calf used to being touched and brushed. Put a rope halter on with a ring, so that it will loosen, and not stay tight. I tie a knot in the lead below the ring, so it will not loosen to the point of falling off.
Either leave them in the corral together, or remove halter and let them back out.
If weaned, or you have both cow and calf in a corral together-
Let the calf drag the lead for several days. Every day spend some time catching it by the lead, brushing, petting and turn it loose. (Still in the corral.)
Once they no longer panic about this, you can tie them up. I give them enough rope to lay down. Tie about an hour or so a day. But don't tie calf with others loose in corral! If not weaned yet, cow will need to be kept close by. This should be a good experience for the calf, not a weaning project.
I eventually lead calf back towards mom and turn it loose.
Calves almost never lead well. Be sure to hold pressure on rope until they step forward, then release as soon as they do.
You don't ever want them to learn to pull away! So make sure you start when you are still stronger than the calf!
I have shown calves with their mothers in cow/calf and they wont lead worth a poop, no matter what. Then the next year, I will pull them out of the pasture, and they lead great! So as a small calf, they don't have to be perfect.
I have found that they do not forget!