I have used a variety of silencers on longbows. Cat Whiskers, wool yarn, fur. Install the silencer of your choice into the string just like a string on any other bow.
For longbows, I do like fur since it just looks cool on a longbow, IMO. You can buy precut fur strips, or if you have a chunk of fur, just cut a strip about 3/8" wide and 4 or 5 inches long, split the string into equal halves, wrap the fur strip around. There's a bunch of videos on youtube...just search for "How to Install Fur String Silencers"