This is goal.
An easy way to get your root angle is to lay a thin straight edge on the root center line under the wing.
Lay the TE stock in place on one wing up to the ruler at the balsa's LE and mark the TE overhang to be cut off.

Mark and cut ends to angle.
Sand and fit at root for a tight fit.

Measure down 4.5 inches and mark.

Use other TE as a guide to mark permanent TE root section angles.
Root section and TE should all be flat on table.

Epoxy the stationary sections to the TE cap.

Lay elevons in place and leave 1/8" between stationary sections.
Mark tip length and cut off to match tip angle.
You'll want to end up with 1/16" gap at tip and center when you hinge.

We need to bevel the LE of the elevons back for down travel clearance since we will attach them with a top hinge.

A razor plane works good here or block sand.
Try not to round the surface except the lower edge if desired after getting the correct clearance first.
Eye ball down the LE of the elevon often so you don't put a curve in it.

Test the bevel for even and enough travel for those inverted spins :)
Once taped, you lose some travel, so go a little more than you think you'll need.

With elevon in position, check along the lengths of the wings for an even transition of the airfoil between the added parts.
The wing is tapered in thickness, with the foam TE about 3/8" at the root and 1/4" at the tips.
You'll need to block sand the TE cap taper toward the tip.
Try to match the rear of the upper airfoil and blend it into the cap and TE stock.

Sometimes the foam isn't molded even with hi and/or low spots, so check entire wing.
When sanding, go slow and keep your pressure on the TE so you don't over sand the foam.
Don't change the natural airfoil, just even it out into the wood.

That's about it for the wing except mounting gear and covering.