How do you have flat top crew cut?

You don't flat top a crew cut. You can have either a flat top or a crew cut. A flat top is a type of a crew cut. Normally a #1 is used on the sides of the head up to the crest. the top is cut in such a way that when styled straight up it forms a straight line across the top. Also normally the very front of the hair (the bangs area) is slightly longer and styled straight up also.