Sometimes borax and sometimes cut off the feathers and glue them back onto a piece of cardboard (could use wood or anything else) why I like about cutting and gluing is you can arrange them flat and near-perfectly and it doesn't take a week or two to dry. Always save some of the shiny hackle at the base of the tail to make it look nicer. Remove the smaller feathers behind the tail fan because you won't even see those. If you use borax (or salt) then trim off all the meat you possibly can, without cutting into the follicles, and work the borax into all the exposed area, all the crevices, make sure it's thoroughly coated in borax. Pin the fan to a board in the shape you like, maybe put some flat kind of weight in the middle to hold the hackle flat against the fan, let it dry at least a week or until 100% stiff so it holds shape when the pins are removed.