There are many ways of how to blend long hair on top short on sides together. Here is one of the easiest and most popular ones:
1. With a comb, lift up the strand of hair on the side below the top section.
2. Holding the comb at an angle toward the head, trim the hair above the comb.
3. Lift up another strand of hair below the previous one and run the trimmer across the comb.
4. Continue doing so until you blend the top into the sides all over the head.