I did my seat cover a few years ago with a One Industries replacement.. it was pretty easy to get the seat to stretch on properly. Finding a staple gun that would actually pierce the plastic seat's underside was the challenge. Eventually I borrowed a 110v electric stapler from a crafty friend of my mom's that worked very well.
As for the graphics kits... I was told to lightly mist the plastic with warm water just before applying the decal, this will help you make small corrections and to remove bubbles. This is how I did my complete kit on my CR250, it turned out very well, and has lasted for several years now. Unfortunately I snapped a rear fender last season so I am only running some of the kit.