How do I make a flat-pack plywood stool with slot-together joints that actually fit?

What the project is

A slot-together stool is a seat with pockets or slots and legs with tabs or cross-lap notches. Every joint width equals the plywood thickness, so the project succeeds or fails on one measurement. Plywood sold as 3/4 inch or 18 mm is rarely that. The software maker's free Easy Shop Stool ships separate files for 18 mm and 0.75 inch plywood for exactly this reason, and its listed sheet thickness is 0.7 inch.

Steps

1. Measure. Caliper the actual sheet in several places. Measure the bit too, and correct its Diameter in the tool if it is undersize.
2. Cut a test joint first. Draw two small rectangles with a cross-lap notch. Make one notch at measured thickness, one 0.003 inch wider and one 0.006 inch wider, and mark them. The right fit slides together with hand pressure and does not wiggle.
3. Draw the parts using the winning slot width. The free project uses the first sheet as a design space and lays the parts out on later sheets.
4. Relieve inside corners. Use Create Fillets. Dog-Bone puts the relief circle on the corner diagonal. T-Bone pushes it to one side and is the option the official documentation recommends when the slot is close to the tool width. Set the fillet Radius to suit the bit you will cut with, and click beside the corner to put each T-Bone on the side that will be hidden.
5. Toolpaths. Pocket any blind slots in the seat first. Then Profile the slots and inside cutouts, and finally the outlines, with tabs and ramps. For accurate slot width, check Do Separate Last Pass; one published joint tutorial uses a 0.015 inch allowance.
6. Tools. The free project uses a 1/4 inch compression end mill for all cutting and a 60 degree V-bit for decoration.

Pro tips

• One maker of a full furniture set made the top slots tighter than the bottom ones so the piece holds together but can still be knocked down.
• Recut the test joint for every new sheet. Thickness changes between sheets and with humidity.
• Joint fit theory lives in the entries on the Dogbone and slot-together gadgets. For handling thickness variation between sheets, see the entry on slot-together parts that fit one sheet but not the next, and for a cross lap test board see the entry on dialing in a cross lap joint.