First, I check the health of the buck. I check his genitals, and make sure he is clean down there. Then I check his eyes, ears, teeth, fur, nails (and clip if necessary), bones (to check for breaks or anything irregular), and just make sure he is healthy and active overall and ready to mate (bucks are usually ready to mate at all times haha).
Then, I check the health of the doe. This takes a bit longer. I weigh her, and calculate her weight so that 14 days later I can weigh her again and see if she has gained an ounce or two. I check her health, similar to what I do with the buck. I check her vent to see if she will be up for breeding that day. Typically, if a doe has a palish pink vent, she does not want to mate. If her vent is dark pink or even purple, she should breed easily that day. I make sure her fur down there is clean and clip her nails if needed.
Occasionally, if I am going to be breeding a buck that occasionally misses or a doe that doesn't want to be bred, I will give the doe and/or the buck Apple Cider Vinegar and this helps regulate the hormones in their body and make them want to mate. Most of the time, your buck will want to breed and will breed easily. But sometimes, does can be cranky and not want to breed.
What do you do before you breed your rabbits?