Use your corresponding piece and line it up with the side of the housing dado that has been cut. To ensure the joint will be tight, slightly lean into the cut line and use your knife to make a nick, remove the corresponding piece and use a square with the knife to create a knifewall.
Use the chisel to pare cut into the knifewall, make sure to take a half width shaving each time as it gives you more control of the chisel. Flick away the waste material, it will split off to the depth of the knife cut.
Lean your chisel slightly away from the dado and tap the end lightly with a chisel hammer. By leaning it away from the recess, you are ensuring that the joint will be tight. Put your wood back in the vise and pare cut to the depth of your chisel chops. Keep alternating between pare cutting in the vise and chopping with the chisel until you are nearly down to depth.
Check your recess fits your corresponding piece, it should be self supporting so you should be able to lift it off the bench slightly and it will hold in the recess.
Use a chisel slightly smaller than the width of your recess to remove the ridge. Start off with one hand around the chisel handle and the other hand tapping the end of the chisel. Make sure your are pointing your chisel towards the sky slightly to avoid it digging in. Once you are near to depth, change to a double handed grip and jab to remove the rest of the ridge. Use the chisel to score the inner corners of your recess to remove the fuzzy bits.
For the final step you can use a router, set this to slightly above your gauge line and use it as you would a chisel to slice away small bits of waste wood at a time, don’t set it to take off larger pieces of wood as this can split deeper than you want it to. Use the knife to remove the fuzzy bits in the inner corners of your recess.
If you have a wooden sole on your router plane the waste wood at the end of the recess might get jammed and stop you going any further, use the chisel to chop down the sides to loosen this waste material.
Test to see if your housing dado fits together, use the chisel hammer if needed. Progress the other housing dados to the same level.