Join the various parts of the vertical and horizontal stabilizers the same way you did the fuselage sides. First, use a straight edge to align the parts.
Then tape the joint and glue it.
After this joint is glued, add the crossgrain bar across the top of the vertical stabilizer. Attach the crossgrain tips on each end of the horizontal stabilizer in the same way. After the glue is cured, remove all the tape and lightly sand the joints with a sanding block. We’re not going to worry about the elevators right now.