If you don't have much hair to blend the unit into cutting the base is going to be the least of your problems. It takes a lot of skill and the right kind of balding pattern to pull off a style like that with a hairpiece. If you are a Norwood 4-7 it is going to next to impossible because your balding pattern extends below the crown of your scalp.

If you do decide to cut the base there are a couple of ways to do it. Some people use scissors but I always use a sharp x-acto blade when I am cutting into the base where it has hair tied in it. It's all about perfecting the technique of holding the base without allowing any tension on the hair and then using the very tip of the blade to cut the base from the underside. The first time you try it it might seem awkward but when you get the hand of it you can safely and quickly trim the lace without having to worry about cutting the hair.