There are 3 main stages of labor. In the first stage, the cervix is dilating and thinning. During this stage, you will see the cow become "loose in the back end", and a clear mucous drainage. The cow may also be somewhat uncomfortable during this stage and may separate herself from the herd.
During the second stage of labor, the contractions start and you may see these on the right side of the abdomen. The calf starts moving into the birth canal, and you will see the "water sack" start to protrude from vulva/vagina. The cow becomes more uncomfortable during this stage, and may be up and down quite a bit, or rock back and forth from side to side. This stage ends with the delivery of the calf.
The third stage is the delivery of the placenta.
Please let me know if you have any other questions, or specific questions regarding one of your cows.