So I cut hinge slots into the horizontal stab and then glued on the vertical stab braces onto the horizontal stab using the reference line as a guide. Wait for the glue to dry, I used Ambroid again. Then just stick the vertical stab in the resulting slot and everything self aligns. The plans show 1/4 inch triangle stock but I think I'll change them to 1/8 inch square sticks instead like I used here. Lighter and plenty strong.
When the glue is dry I test fit the stab assembly into the slots in the fuselage. If it is a little tight or needs a bit of adjusting use a sanding stick stolen from your wife's drawer of fingernail maintenance supplies. You'll find all sorts of useful modeling stuff in that drawer!
Don't worry, just tell her I said it was OK to help yourself!