Since I hadn't used that router blade yet, had to do some work to get it flat and get rid of the mill marks from the factory. All better now!

Quick tune up of the 3/8" chisel.

Mirror finish!

Either use a scoring marking gauge or a sharp knife to score the lines to help prevent tear out. Also not pictured here is mark the depth line on the end grain.

Nothing worse, or quite frankly dangerous, then chasing your piece around the bench or putting your hands near the pointy bits of the tools.

Adjust your router plane's fence to be on the edge of your scored lines. Note to self, buy a 3/8" router plane blade (or find it, since I swore I had one...)

Since we're doing a 3/8" dado, grab your recently sharpened 3/8" chisel (you did sharpen it right?) and lightly score a line. Keep the bevel facing the waste area and don't go pounding too hard. You really just need light pressure to sever the wood fibers and you can clean out as you go deeper

Take light passes and go progressively deeper as you go. No need to take 1/8" passes. The more you force it and rush the more you'll risk tear out and losing control and losing your line. Also make sure to keep pressure on the front/back as you get towards the end of the work piece to keep your plane from diving off the end.

The shape of the router plane blade won't allow you to get all the way to the end of the cut (on this view, if you're pulling the router plane towards you). You'll have a ramp along the back wall. Grab a sharp chisel and pair away the waste to get it back flush with the other side, or adjust your fence on the router plane and take more passes pushing away from you towards the end of the cut.

Yay, grab a cold drink and celebrate your hand tools stopped dado cut. Normally you would cut down the piece to final length and get rid of the end milling from the lumber yard, but since this was a piece with knots I'm going to end up tossing - I just left it there.