There are a number of ways to fold hackle feathers. Here’s an easy one.
After stripping away most of the lower fuzzy fibers from a hackle feather, get the bare stem firmly secured in the jaws of your tying vise with the shiny side of the feather facing forward or toward you. While holding the feather’s tip in your right hand, bring the fingertips of your left around the feather and squeeze them together to form an “O”. Then, pull the feather down through your fingertips bringing the stem into an upside-down “U”. Pull the “U” through your fingertips. In other words, pull the feather through your fingertips rather than pulling your fingertips over the feather. With just a couple of passes, the feather should be well-folded.
Of course you can do this with the feather secured to a hook. Again, it’s bending the stem down through your fingertips that’s the trick. Once you’ve got the feather folded, then it’s easy to wrap and secure in order to create a nice, even, swept-back hackle collar.