First apply a half hitch but DO NOT pull the slack out of the thread. While keeping your fingers spread with the thread around them, rotate the piece of thread that is attached directly to the hook around the hook shank in a clock-wise direction. Two or three rotations are all that is required. Now pull the slack out of the thread removing your fingers as you go just like you did for the half hitch or use your scissor's tip to steer the loop into place while you pull the slack out of the thread. The illustrations should help you understand this process.
Hackle size is determined by the length of the individual fibers in the usable portion of a feather.
You are probably wondering how to measure the fibers and also, what part of the feather is the usable part. Determining the length of the individual fibers is fairly easy. Just fan the fibers by forming an arc in the feather stem. Then gage the length of the fiber by measuring from the stem to the end of the fiber. There are special tools on the market today to help with this measurement or you can use the hook to make your comparison.