You don't need a jig.
Lay them out like below, then when you cut them out, just barely leave the lines.
Then get a snug, not tight, fit by sanding a little. Ya need a little gap for glue, maybe 1/64th.
Don't force the joint together or it will split the board.
I use a 1 inch belt sander and can slide the belt inside the cuts and slowly clean them up.