As the pic shows, I have my string marked above and below the nock. Take a dab of string wax and put it on the center serving. I then take a piece of serving thread and fold it in half and lay it in the wax. The wax just helps hold the piece of thread in place.
Then I take my thread for my tie in nock(I prefer BCY 3d), lay a tag end on the center serving, make 5-7 wraps around the center serving and the tag end. Once I make my wraps, I insert the end that I used to make my wraps through the loop, and pull through. Believe you fly tiers call that a "nail knot".
There are the finished tied in nock points top and bottom