If this is like all of BJ's other planes, there is likely a Laser CNC cut jig that you use to locate where to cut the slots for the G10 horns. It is probably in the bag with the other laser cut plywood parts. He probably has a jig for ailerons and a separate one for the rudder and also elevator. The elevator jig likely has a hole which fits over the forward tube in the stab and helps locate the slot for the horn. That one is important to find and use. Check the BiSide thread, I'd bet that this one is very similar in construction method and fixtures.