That's easy. Use 30 minute epoxy so you have some working time. Cut off the covering where the glue goes if you need to, then glue it up and stick it in the slot. For the horizontal, use a bubble level to make sure the wing is level. Don't depend on the landing gear for this; use some books or other solid material to block them and the back of the fuselage in place and use some weight on top of the fuselage to make sure nothing moves. Then use the bubble level on the stabilizer and block it into place or use packing tape to hold it. After it cures, put the vertical stabilizer in the slot. Your eyes will get it within just a degree or two if you stand back a little and check it for square, but if you really want to get it perfect use your level to get it vertically plumb (check both sides so you aren't fooled by any warps or taper) assuming that your horizontal stab is still level also. Then use some packing tape to hold it in place while it cures. Alternately, do a Google image search for a square you can print out and glue it onto cardboard. You can cut out the point of the right angle to allow for your fuselage and then use it to get your vertical square.