Headwall should be treated the same as you would with asphalt shingles, using a metal flashing that extends at least 4” onto the top course of shingles. If a beauty course is desired to cover up the flashing, cut shingles horizontally just above the nailing strip and apply over the flashing.
With a generous bead of sealant under the nail strip, nailing through the flashing should be avoided. If a nail is necessary, be sure it passes through the sealant on the back of the shingle at each nail location, and apply a colored sealant to the head on all face nails.
Beauty courses will rarely line up with adjacent field shingle courses, so demonstrate the option to your customer first, as some may prefer the flashing to an offset beauty course.