I originally typed this up as a PM to someone, but I figure that some of you out there may not know how to do this and may find it useful.
The firststep is to cut a decent length of serving, I usually cut around 10 inches or so, even if I'm only tying a small nock set. I just like to have a lot of extra to play with.
Second, make a loop with the serving where you want one side of your nock set to be [top or bottom],holding it in place with one hand.
Now, using the longest piece of the serving hanging down, wrap that four to five times around the loop you've made, moving in the direction of the loop. You can wrap it loosly to prevent the serving from slipping, but make sure to pull it as tight as you can get it once you've finished wrapping it.
Hold this in place with one hand and again using the longest piece of serving hanging down, feed the end of the serving through the eye of the loop.
This time, while pulling the end of the serving you just fed through the eye of the loop tight, pull the opposite end of the serving tight at the same time.
This will pull the eye of the loop underneith the serving you wrapped over the loop. Remember to pull the longest end of the serving tight while you're doing this.
Now, use a pair of scissors or fingernail clippers to cut both ends of the serving.
Use a lighter to burn the frayed ends, being careful not to burn your string. Now repeat the process above or below this served portion and then tie your loop on either side of these served portions.
One thing you may want to do differently, something I recently learned from Matt/PA is to tie the bottom serving 2/3 longer than the top. This will pull your loop down level behind your nock rather than slightly above it.
Hopefully this helps some of you! If anyone has an easier way to do this, let me know!
Enjoy.