The purpose of having a serving on the string is to protect the string from wear. The serving needs to be long enough to hold it when shooting and so the serving will touch your arm while holding the bow.
You can apply serving free hand if you want, but it's much faster and easier with a serving jig. It's also easier to apply the serving while the bow is strung. It's almost impossible to apply serving if the string is not under some tension.
The first thing you need to do is cut off about 6" or so of serving thread and put that to the side, and protect it from the cats.
The next thing I do is pull some thread from the server, and lay it on the string above the arrow rest so that the loose end of the thread points toward the lower limb of the bow. I hold the serving thread against the string and wrap it around a few times. Tighten the server so that no more thread comes out of it.
I keep wrapping, holding tension, and keeping the strands close together, until I've got about 1/4" by my trusty eyeball. Then I pick up the loose end and flip it over so it points toward the upper limb of the bow.
Then I continue wrapping until the server butts up against the string, and the string fits into the groove on the server.
At this point, I loosen the server wing nut a little to adjust the tension. When I turn the server around the string, I want it loose enough so that I can turn the server with ease, but not so loose that the string comes out of the groove. Use your own judgement. Sometimes my server gets a little loose as I'm turning it around the string, so I have to make adjustments to the wing nut as I go.
Keep turning the server around the string, making sure the thread is close together, until you get it to where it's long enough to touch your arm when holding the bow, or however long you want it. Then pull the server off the string. Take that 6" piece of thread I had you cut off earlier, make a loop with it, and lay it on the string so the loop is pointing toward the lower limb and the loose ends are pointing toward the upper limb.
Now wrap the serving thread around the loop so it holds the loop to the string. Continue wrapping for another 1/4" or so. There should still be some loop hanging out the end. Cut the thread from the serving jig so that you've got at least 4" of thread to play with. Stick the end of that thread through the loop.
While holding the thread with one hand, pull the loose ends of the loop with a pair of pliers. I just jerk it through.
The loop will pull the thread under itself. Once it does, pull it tighter with the pliers.
Do the same thing to the other end. You don't want to pull it so tight that it bites into the string, though, because that could shorten the life of the string.
Some people like to put glue on both ends of the serving to keep it from unraveling. I don't do that, because it makes it harder to take the serving off later in case you need to replace it. If it unravels a little, it's no big deal to redo the serving.
Now take off the twisty ties, trim the loose ends from the flemish twist, and wax the whole bow string, but rubbing wax on it, then rubbing it fast with your fingers until it feels a little hot from the friction.