I measured the hinges so they are a correct fit to the flap spars, trimming off the excess. Clecoed the hinge to the spar at the correct depth and drilled the rivet holes on both spars/hinges. I made a template to check the correct depth of the countersinking required on the spars(see pic), nothing fancy, but it'll do the job. I then did the countersinking on both spars.
