I use a large (red) handled x-acto knife. Drill a hole in a block of wood to make a tight fit for the handle and clamp the block to a bench. stick the knife in the hole (blade up, obviously), skewer the quill of the feather on the knife in the center of the quill and near the big end, then pull the feather towrd you, guiding (not pushing) the feather with the other hand. When it is split to the end, flip it around and cut what little bit you didn't get originally. I find it a LOT easier to cut the feathers this way than to lay it on a table and try to cut down the center of quill. You may not get every feather to cut right down the middle to the end using this method, but you'll get most of the feather split right down the middle and ruin a whole lot fewer than when you're trying to hold the feather in one hand and the knife in the other. I suspect that it's also a lot faster.