With a block plane, plane one surface of the fretboard plank as flat as possible. This will be the back of the fretboard (the gluing surface). Fretboard planks from guitar shop suppliers are usually prepared on a machine. So if the plank already has a flat surface, better leave it alone or, if necessary, go over it with the scraper.
Attach the back of the fretboard plank to your workbench with two long strips of doublefaced tape. Then plane the top surface of the fretboard until it has a uniform thickness of 6/32" to 1/4" (5mm to 6mm).