In preparing to attach the stool’s top to the leg frame, I first measured, marked, and cut angled tenons on the tops of the legs. I used the scale drawing again as a guide. Here’s a photo of an angled tenon before I finished cutting the shoulders at the long ends:

That was a fairly simple task, even though I was cutting the tenon before the mortise. To make sure that the tenon fit, I chopped a mortise in a piece of scrap and test-fit.
Then I assembled the frame and double-checked the distance between the tenons, and carefully laid out the lines that I’d chop against on the stool top. Then I broke my mallet chopping the mortises (as described in the preceding post), so I had to wait for the glue to set on the mallet repair until I could get going again. When I did, I was excited to see a test fit.
I was not so excited to discover my latest boneup:

Arrgh. I had chopped the mortises on the wrong side of the lines. I have got to get more careful with this. It’s, what, the third time I’ve done that in two months? I was pretty discouraged, so I took a break while I tried to figure out what I would do about it. Also, I was hungry.
Then I went back to the shop and cut out a piece of beech to the same thickness as the tenons. I inserted this into the mortise, and then extended the mortise (this time in the correct location) by chopping against the side of the inserted section:

After doing this for all four mortises, I had the following:

And then I did a test fit to make sure that I’d actually chopped them in the correct place this time:

Phew. It fits, and there’s only a small void showing on the underside of the stool. But if left and glued up like this, there would probably be a tendency to rack, because the legs could conceptually slide around. Therefore, I had to fill in the originally-cut side of the mortise.
I sliced off a small section of the piece of beech I used to register the chisel when chopping the correct mortise, and glued that on one side where I’d originally chopped the mortise. Then, on the other side, I inserted the section again to “clamp” it in place as the glue dried:

After each mortise got its repair, I did another test fit. Incredibly, it seems to have worked. Now I just have to finish off the edges on the components and glue up.