In this stepstool, the bulk of ensuring that the folding ability works correctly is concentrated in the legs. There are four legs with two different angles, and an angled housing dado. It’s important to keep everything in order, or it will quickly degenerate into firewood.
The first thing is to scribe the angles into the legs. To do this, I use the protractor adapter for my combination square, and then knife in the line.
Then, I use a bench hook and my trusty Disston hybrid saw to cut the angle. A bit of work with a plane will smooth it further.
Eventually, all of the legs are cut to the proper angle. Keep them in order!
This is not a particularly difficult or complex operation. It does not require a mitre box or some such. The difficult lies in making sure that the right angles go in the right place, especially since the pieces are so very alike. Remember that you should end up with parallelograms. If you have a truncated pyramid shape, you’ve gone off the path somewhere. Do check yourself before you start sawing away!