If you have a router, buy a 1/4" slotting (spline) cutter with a ball-bearing guide. This works better than dowels or a biscuit. Cut your spline about 1/8" norrower than your slot. If you buy 1/2" bit, it will cut 1/2" deep into each of your boards. This gives you a 1" total slot when the two boards are put together. Cut your spline 7/8" wide from 1/4" plywood to allow for a 1/16" glue line on each board.
Start and stop the cut 3" or so from each end. This hides the spline.
A slotting bit is a great tool to have in the shop.
If you don't have time to do it right the first time, when will you have time to do it over?