Make your router think it's a jointer
If you don't have a jointer or if you're just getting into woodworking, a router table with fences and a straight bit can suffice for edge-jointing short boards.
We added an edge-jointing function to our router-table fence by simply clamping a piece of plastic laminate on the left-hand, outfeed end of the fence. Use sandpaper to ease the edge nearest the router bit, so it won't catch your workpiece as the board slides past. As seen in photo, we used a steel rule to align the laminate with the cutting edge of a straight bit mounted in the router.
Set the bit high enough to trim the entire edge of the board in one pass. Then, turn on the router, and move the board across the table from right to left. You'll remove 1/16" with each pass, and leave a perfectly straight, square edge. Repeat the procedure with a second board, and the two pieces can be glued together without a gap anywhere.