Get yourself a piece of dental floss. Tie one end to a piece of your marabou, then tie the other end to your string, a couple inches down from the top loop, or a couple inches below where the string contacts the top limb on a recurve. You don't want the feather dangling around real loose, so only leave it hanging about 1 - 1 1/2". You also don't want a whole marabou feather waving around. Just use enough that a slight breeze will move it.