The classic way would be to drill the ends of the slot, as @steve6 said, then use a straightedge and sharp knife to score a line along the two edges of the slot. Do the scoring on both sides of the wood if the back side needs to look clean too.
The scored line will serve as a precise guide for precisely aligning the edge of the chisel. The idea is to use one of your wider chisels, say the 1”, to cut down partially along the scored lines. Hold the chisel with the beveled side of the edge toward the interior of the slot and the blade held perfectly vertical. Make the edge drop into the scored line. If the chisel is sharp, you can just use firm pressure to cut down into the wood. You don’t want to go all the way through, as that would likely split the piece. Cut down along all the scored lines, then use your 1/8” chisel, held with the bevel down, to cut out the top layer of wood that the larger chisel has freed.
Eventually, you’ll reach the depth of the side cuts and will have to use the 1” chisel again to deepen the side cuts. Keep repeating the cycle til you’re through the piece.
Use the 1/8” chisel to square off the round ends of the slot (you may need to tap it with a hammer to cut cross-grain like this.