I pre-paint the parts roughly the colour that the carpet will be. Once dry I brush on a thin coat of Elmer's white glue (as seen above most thin it, but I use it straight). I work in relatively small sections at a time. Thanks to a tip I got from this forum (though I forget the member's name) I use an old salt shaker to apply the powder. Sprinkle it liberally on the glued area, and if possible tilt the part around a bit to flow the loose powder over the glued area a couple of times. Then dump the excess onto a sheet of paper for re-use. Once the glue has set, invert the freshly carpeted part and 'tap off' the loose excess.