Step 1: Material
This way you can mark the dimension on one piece and cut all of them at the same time.
After cutting and drilling all the pieces group them in four groups, one for each stool.
Step 2: Gluing Boards
To make a better junction I drilled 8mm holes and I plugged and glued two wood dowels for each couple.
I kept all the boards strongly pressed with a belt and I kept them flat on the table with a board and a pair of clamps. This is necessary since the strap tension tries to lift them.
Then wait the glue to dry.
Step 3: Drilling and Dowels
This is the simplest way to obtain a strong connection, although it's not effective like a good dovetail.
There are very useful metal markers, little cylinders with a spike at one extremity. Drill an hole on one side, insert the cylinder, place the opposite piece in the right position and press it against the spike, or use an hammer. You will have the exact center for the next hole.
Add glue in the holes, on the dowels and on both surfaces, just the right quantity, you will learn it after a few tries. Then place parts in contact.
Step 4: Gluing
Pay attention that every connection is making 90° angle. Otherwise you have to stretch a cord to force pieces during gluing process.
This method is very simple, check diagonals and hook the cord on the longer one, then place a little wood bar in the cord and twist it until the diagonals are of equal dimensions.
Step 5: Still Gluing
Step 6: Assembling
Use self-locking bolts or add glue to avoid loosing.
Step 7: Straightening Out
Step 8: Finishing and Completing
Enjoy and be proud of your new creation!
Step 9: [UPDATE]
Other notes in the drawing are referred to the original project.