This is the idea.
We want to install the front spars on the wing bottoms.
Note that although the wings match up (LE & TE) the molding holes and recessions do not!

Measure back about 2 inches from the point that the nose of the root should be.
(there's about 1/4" missing)
Make a mark at the root on both wings.

Now measure in 1.25 inches from the LE at the tip.
This is about the 1/4 chord location and is also the thickest section of the airfoil.

Lay a straight edge or yardstick between the marks and carefully draw a line with a pencil on each wing half.

Put the wing bottom-up on a piece of soft foam. The humped root end can overhang your table for a steadier support.
You can carefully clamp the straightedge down on your line. (or offset the correct amount for a router guide)

Using a Dremel router is the best for cutting the slot if available.

Use a 1/8 inch wide bit and make 3 shallow low speed passes each deeper than the last with the final at 1/2 inch deep.
This removes the cuttings from the slot as you go and is a lot easier than picking it out later!
Just run the slots right off the ends of the wing halves.

The slot can also be cut with a razor knife that has a piece of tape on it for a depth gauge at 1/2 inch deep.
Make one cut, move the straightedge 1/8 inch and cut again.
You'll have to dig out the foam between the cuts if you use this method.
A bamboo skewer stick works good for this.

Another method is to make a 1/8 inch wide notcher with a piece of hard balsa.
Round the end and glue a thin strip of medium sand paper to it and mark it to the proper depth of 1/2 inch.
Keep it up against your straightedge and don't try to rush.

Sand the spars ends angled to meet flush when joined.

Test fit in place and assure proper joint fit and depth.
It seems easier to glue the spars in after wing joining but maybe that's just me.