I made some for ash boards, red oak, and aromatic cedar, in both cases I ripped them out to slightly larger than 3/8" and planed/sanded them down by hand. Bad idea. Cut them to 3/8" squares, then do jsut as Pat said.
I've also used the method you're refering too. Hard, dry woods don't work well with that method. I could do it with wet hickory, but dry oak and I'd be going nowhere. Hand planing 3/8" squares is easier suprisingly, at least for me.