Well, you can kinda divide birth into stages.
stage 1: Might last up to 24 hours. Restless, pacing, going off by herself, tail twitching, nervous, maybe kinda looking for privacy/isolation. you might see kinda a mucus plug dangling.
stage 2: she's trying to deliver. water bag out. then toes. if toes are right side up, it's frontwards. upside down and it's backwards---or very rarely---upside down. then comes a nose.
The cow might want to be left alone, if she's the nervous type, every time you get too close she kinda has to sorta start over.
She might be getting up and down a lot anyhow. Circling, then laying back down. If it's coming normal and she's making progress, leave her alone. When calf gets out, make sure it ain't got a bag over it's head, that it's airway is clear. maybe stick a straw up it's nose and make it sneeze. The easiest way for it to breath is like a lion in front of the bank---upright, hind legs ahead on either side, front legs forward. Sometimes they dumped on their head and it kinda gets twisted under their body--you wanta rectify that.
stage 2 might take an hour or so. Don't freak out if it's head and chest are out and it's breathing/bawling---and it's kinda natural for some cows to take a break about now---umbilical cord should still have it on life support.
Some cows take longer than others, and take breaks. Usually lay down once they get serious. Generally on right side, paunch in on left---this gives birth canal a straighter shot.
stage 3 is delivery of the placenta. might take up to 24 hours.
one purpose of the waterbag is to dilate cervix. If feet are out and you gotta intervene and water bag is still intact, slash it open, it's job is done.