To take the old string off, pinch the string a couple of inches above the yo-yo and spin it to the left until the string opens up. Slip your fingers into the gap in the strings and slide the old string off. Now take your new string and gently untwist it until you can insert your thumb and forefinger in the loop at the bottom. Slip this loop over the yo-yo and onto the axle. Keep a little tension on the string so it retwists correctly. To double loop do the same procedure, but after you slip the loop onto the yo-yo, twist it once and put it back on again. Generally the string should be short enough that you can impart good force on a throw. One guide is to tie the finger loop at approximately your belly button. Bear in mind that everyone prefers something different on string length. Find out what works for you.