i use clamps on the horizontal and align it with my eyes and a triangle. I use epoxy to glue major sections like this in place. for the vertical stab after the horizontal stab is dry. I use 2 triangle placed on either side and run a piece of masking tape from hor stab tip over the top of the vertical stab to the other side and secure it to the other hor stab tip trapping the vertical stab (rudder) in place. double check with triangles to ensure the vertical stab is at 90 deg. I'll leave the triangles in place while the epoxy is curing so I can glance over to make sure I'm staying true.
follow the instructions for left and right alignment of the vertical and horizontal stabs.
It's not in the instructions, but after you have secured the verical stab in place glue a piece of triangle stock at the base of the vertical on either side for some reenforcement.
Your going to love the LT-40 I learned on one, have 1 stock, and 1 heavy modified.