Shaped a thin straight chisel from half inch wide clock spring to form a blade 1/4" wide. Made another from same spring material with a diagonal blade with a 1/32" cutting edge. The wide blade cuts the top and bottom of the slot and the narrow one cuts the side walls and cleans out the debris. I cut the slots after most of the wood has been planned close to final dimensions but still square so any buggering of the wood at the outside edges goes away in final shaping. But the most important thing for me was the practice on a piece of scrap maple until I was confident in my tools and their use. Didn't take long.
