Developed this project while teaching a woodworking course to a group of young adult beginners.
Here's how they made these wooden stools.
First we designed the stool and laid out a full scale joiner's rod drawing. Then everyone practiced the necessary skills by making mockups out of scrap wood and doing destructive testing of their practice joints.
Seats are 2" pine, legs are 1 5/8" square ash, stretchers are 7/8" square ash.
To prevent weakening the seat too much, leg mortises are laid out so that each hole is in a different line of grain. First a splay line is drawn on the underside at 22.5 degrees to the line of the grain, then a compass is used to construct a perpendicular bisector for the other splay line. Centreline circle of legs is 2" in from the edge to give plenty of meat to prevent splitting. Mortise holes are drilled with a 1 1/4" auger in a brace. The angle of the brace and bit is guided by a bevel gauge set to the angle on the joiner's rod held on the centre of the splay line. A helper eyeballs the brace alignment from across the bench.
Tenons on legs are cut with a backsaw and finished with a chisel. Tenons on stretchers are shaped with a spokeshave. For final sizing, tenons are 'mulleted' with a hole drilled in a block of hardwood. Rubbing a pencil inside the hole helps to mark the high spots for fine fitting.
Once the leg tenons are fitted and adjusted to their final position, the shoulders are scribed and recut to the angle of the seat using a backsaw on a bench hook.
Legs are planed octagonal in this jig - a piece of 2x4 with a v-groove down the middle. The lines to plane to are marked with a spar gauge.
The tenons will be locked in place with wedges. They have enough power to split the seat, so they need to be laid out perpendicular to the grain.
Kerfs for the wedges are cut with a rip saw.
Ready for glue up. the underside of the seat has been beveled with a spokeshave so that it doesn't look so heavy.
Because of the angles, all of the joints have to slide together at the same time in order to assemble the stool during glue up. Practice with a dry run before applying the glue.
Lightly glue all the tenons and then assemble quickly. Dip the wedges in glue before driving home.
Stretcher wedges must also be oriented perpendicular to the grain of the leg to avoid splitting the legs during assembly.
After the glue has dried, trim the tenons flush.
And plane the legs and seat smooth with a sharp plane.
Finally, trim the legs to length. Mark the length and angle by putting the stool upside down on a perfectly flat surface like a table saw. Use a framing square held vertically on the table top to mark around each leg. Cut with a backsaw, test for high spots by standing it up on the table saw.
Sand lightly, apply a couple coats of wipe-on finish, and they're done.