If you've got your nut files coming, then yes, it's just a question of cutting down to size, filing and sanding (wear a mask as bone isn't a nice dust to inhale). You don't want to file the fretboard side of the slot at all, or you'll affect the intonation, albeit very slightly. Just do enough to remove any glue from the front face of the slot, and concentrate any filing on the floor of the slot, (and the rear if you must). Better the blank is a bit wider than the slot, so you can then sand the blank down to fit the slot after the slot has been filed.