Once you've acquired ten or more bolts for you crossbow, you’ll want to find one walker that you can single out from the rest. Make sure he’s the only walker in the vicinity to ensure you don’t get bitten by other walkers. Once that's done, simply let off ten or more bolts in any part of his body other than his head/neck. Try not to shoot bolts in the same part that you've already hit, as most of the time the bolt will fall off of the walker. After your 10th bolt the achievement will unlock so long as it did not kill the walker.