We want to flatten the bottoms and square the center for joining.

We'll want to achieve a total sweep angle that equals about 19 inches.
(lay one wing LE straight on table and measure back to other wing tip LE)

Start by rough cutting extra bottom foam off.

Block sand with medium then fine paper until nice, smooth and flat.

Check it this way...

and this way...

and especially this way!

Try to remove the least amount of foam needed to get the root vertical and achieve the desired sweep.
We don't want a shorter wing, only a root junction that's square to the wing bottom.
(No dihedral necessary)

Use edge of building board as a guide to help stay straight and square while sanding edge smooth.

Finished joint and bottom surface

A little angle change at the root becomes a large one out at the tip!
Keep checking your sweep, but don't worry about it too much.
A little off (18" to 20") shouldn't effect it's flight much, only it's CG location some.
19" total works out to about 20% LE sweep angle.

(BTW - With 18" of total sweep for V1, the CG was right on the rear spar.
V2 has 19" and a 1/4" wider TE and it's CG is around 1/2 inch behind the rear spar.)

Finish root by slicing off that annoying nub at the TE
Keep the cut square with the bottom and in line with the TE.

Now line up the TE with your building board edge guide and sand the foam TE nice and flat.
Eyeball down it often for straight and square. No curves allowed!

Here's what we're looking for.

Hold the root LE and TE down flat on the building board and check the wings for true.
A little dihedral is ok (slight curve up at tips from molding).
Any twist difference between wings is not if due to uneven root area bottom.

Eyeball from the front, sides, back, etc.
Be sure that you have no wangs, twists, or warps.
This is important so get it right now!

Also, measure the tip LE down to the table (if raised) on each side as well as the tips TE height.

Hopefully, you might get a little natural wash-out (TE at tips higher than LE at tips)
If the wings are not even and you are sure that your root bottoms are flat and level,
you can pull in (or pull out) a little adverse twist (or even add some wash-out) when taping.

Fine sand any mold marks, bumps, etc. and your wing blanks are done!