I used pine to make the neck. This is not the normal wood for guitar necks, but it is cheap and you make need to have a few gos before you get it right. The piece was around 28mm thick, 106 mm wide and 738 mm long. The two sides you will be machining should be as parallel as possible.
Fix the neck blank to the spoil board - with clamps or double-sided tape whatever works for you - make sure the toolpath doesn't collide with your clamps. The blank should be aligned centrally with the spoil-board.