What ever you choose make sure it has a solid core to it or the rope will compress over time and may either break or slip through the jaws of some releases.
Here is a picture someone else uploaded a while ago on tying a rope loop. I don't do the extra serving in the middle as I find the knots hold their position well enough that I can just position them where I want them and they cradle the arrow without pinching in on it.
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SuperX