You will need these materials for each chair:
Approx. 14' of 2" x 4" or similar
Enough 3/4" material to make two planes, approx. 18" x 16" each -- I used maple flooring, but you could use plywood or other material
16 #10 x 3-1/2" galvanized wood screws
Tung oil or finish of your choice
You will need these tools:
Bandsaw, jigsaw, or circular saw
Random orbital sander
Assorted bar clamps
Step 1: Cutting the Frame
The legs are all 24" long; to ensure they all hit the ground evenly, and your chair doesn't rock, set up a stop on your chop saw. Clamp a block 24" from the blade, as shown, then butt your workpiece against the block and chop. This makes all the pieces exactly the same.
Next, lay out the angles for the legs. The back of these chairs is too upright -- the next version will be more laid-back for comfort. I would recommend cutting the back at a more extreme angle, perhaps by using 2" x 6" material for the back legs. The angles are really up to you -- I tapered from 3-1/2" at the top to 2-1/2" at the bottom for the front legs.
To mirror that taper on the back legs, maintaining visual balance, measure 1-1/2" in from the long edge of the board at each end. Then run a straightedge from your mark to the opposite corner and strike a line. Repeat for the second mark. You will now have two opposing tapers, intersecting at 12". The intersection will be the top of the seating surface.
Mark a line at 14" from the bottom on the front leg -- this will be the top of the seating surface in the front. A fall of 2" will provide a nice, deep recline, and 14" off the ground is a low-slung, loungy seating height. Cut all the tapers with a bandsaw, jigsaw, or circular saw. Run a belt sander over the cuts to smooth out any ripples from the cut.
The horizontals are also 2" x 4"s, ripped down to 2-1/2" wide on a table saw. The seat horizontal is about 17", and the arm horizontal about an inch longer. Lay them out on top of the legs and trace notches out, giving the horizontals at least a 3/4" seat. Cut the notches with a bandsaw or a jigsaw, making sure to cut a little small for a tight fit.
Step 2: Assembling the Frame
To assemble the frames, lay the pieces flat and plug the pieces together, liberally applying glue to all mating surfaces. Counterbore, pre-drill, and sink the screws. You'll notice I offset the screws towards the outside face of the frame; this was to allow room for the router joints that hold the seat and back in place.
Use clamps to align everything. Let the glue dry for a few hours, then sand everything flush with an 80 grit paper on an orbital or belt sander. HIt the joints carefully, as that's where misalignments will glare the worst. Polish it up with 100 grit, then 120.
Step 3: Routin'
Mark out your slots; mine worked off of the width of the flooring I had, so they were a multiple of 2-1/2" long. The seat, for instance, is 15" deep, so the slots are 15" long by 3/4" wide, centered in the depth of the seat horizontal and centered front-to-back. Make yours according to whatever seating material you've chosen. Also, make sure your marks are mirror images of one another, so the two opposing frames match up once assembled.
Keep in mind that the back can only accomodate about three or four boards before the groove gets too close to the edges of the wood, making it structurally unsound. This would be another argument for making the back out of a wider piece of material.
Find some thin material to use as a guide. Rip it to straight strips on a table saw. Screw or clamp into place along your marks. Use a plunge router and top-bearing bit to cut the slots about 3/4" deep. Square the corners with a chisel.
While I had the router out, I used a chamfer bit to add a nice round-over to all the edges of the frames, smoothing the frame out to the touch. This is totally optional.
Sand off any pencil marks and give the frame a final once-over.
Step 4: Flooring
Cut the tongues and grooves off on a table saw to get nice, clean edges that will glue easily to one another. Typically, once pulled up, the tongues and grooves will no longer fit reliably together because of the trauma sustained during demolition, so it's better to just slice 'em off.
Run a sander over them to take off the grime, but not the finish, and cut to 18", using a stop to ensure congruency.
Step 5: Assembly
Put more glue in the other slots and hammer that frame onto the seat and back boards. Clamp the hell out of it. Pop a few small, trim-head screws through the frame and into the seat at the four outside corners for extra strength.
To bring the back up to a better height, I put one cheater piece in -- glued and screwed -- and a cap piece, notched over the side frames on the bandsaw. You can finish it however you'd like -- cutting it off flush with the top of the chair frames or bringing it up further. Rout and sand the edges of the seat and back for ergonomics.
I added a few hand-rubbed coats of tung oil on the frame but not the seat. Use the finish of your choice.
Sit and enjoy!