I've been looking into the same thing myself. I haven't had it done, so I'm a little nervous about giving advice on something I have yet to experience. But, so far I've been told to find a reputable vet in the area that has done ear cropping on pits before. The last thing you want is a hack job from some quack.
Make a list of vets in and around the area that you live and give them a call. Shortlist the ones that have done it and are within the price range you are looking at and then go visit them in person. You could even ask those that you see if they have any photos for you to look at.
Each vet will have his/ her own style and method: see which one you like most and just be prepared for post- crop care.