You will need these materials:
Approx. 40' x 3/4" recycled oak or maple floorboards
Approx. 13' x 1-1/2" x 1-1/2" yellow pine or similar
20 1-1/2" drywall screws
Dowel scraps for screw plugs
Boiled linseed oil
You will need these tools:
Table saw or circular saw
Drill/driver and assorted bits
5-6 3' pipe or bar clamps
3-4 1' bar clamps
Orbital or belt sander
Plug cutter (optional)
Step 1: Lamination
Go over the wood with a metal detector (optional) and close visual inspection to find any old nails that might keep the boards from locking together. Lay out the pieces, staggering any running joints, until you have an arrangement that will end up being about 48" by 30" (or however large will best fit your space). Coat each tongue and each groove with glue, and lock together loosely, resting on a few of the pipe clamps. Alternate your clamp arrangement above and below the boards to help prevent buckling. Evenly apply the pressure up and down the row of clamps until all the joints are tight. If the surface is wavy, clamp a 2" x 4" across the boards and to the workbench to force them flat. Clean up glue squeeze-out with a damp rag. Let dry for twenty-four hours.
Step 2: X
Whatever your source stock, rip it down on the table saw to a consistent width -- 1-1/2" by 1-1/2" should do nicely. Cut two pieces long enough to span all the boards in your table top on the diagonal. Lay your X pieces out on your table top and trace the angle at which they intersect onto one of them. Match the angle on your chop saw and cut, subtracting the width of the matching piece so that all the legs of the eventual X will be the same length. Put a 5o miter into the ends of the X pieces. That miter is what will make the legs angle out.
Step 3: Legs!
The X brace and the legs are attached with simple lap joints, meaning half the width of the material has been subtracted from each piece, so that when glued together they are the same thickness. Measure out your cuts with a square and a pencil, then use a bandsaw and a chisel to cut away the joints.
Once clean and tight, glue and clamp the joints together. Use a polyurethane glue (like Gorilla Glue) for superior strength. It is crucial that these joints are tight and strong, as they will take the most stress. Keep in mind that polyurethane glue must cure in the presence of water, so dampen the matching surfaces before applying adhesive.
After the glue has dried, use a hand plane to smooth out any inconsistencies in the joints. Polish up with a power sander, removing any glue spots and burnishing the joints together. Lay out your X, centering the two "L" shape legs on the long piece of the X brace, and glue and screw together.
Step 4: Bracing
As you can see in the photo, the ends of the flooring run wild -- you will trim them later. Find a rough center and align the X so that it crosses every floor board. Countersink, pre-drill, and screw the X into every board. Hitting every board is crucial; this will ensure stability in the long term, keeping the table top from twisting and bucking too much with moisture and temperature changes over time.
To brace the legs, lay a straight edge across them, then use an angle finder as shown. Transfer the angle to the chop saw and cut one brace for each side. Glue and screw mid-way down the legs. I counter-sunk the screws and then used a plug-cutter to make dowel plugs to cover the screw heads. This step is optional, but adds to the finished appearance. Lacking a plug cutter, use manufactured dowels, but be aware the color and grain match will probably be off.
Now that the top is braced and sturdy, run the whole table through the table saw to trim the tongues and grooves off the long edges and even up the short edges. Use a sled to make the cross cuts. This can also be accomplished with a circular saw -- just be sure to clamp down a guide board so your cuts are nice and straight.
Step 5: Finishing
Polish up the legs with a power sander, then ease the edges by hand to they aren't sharp or splintery. Make sure the doweled joints are sanded flush. Wipe with a damp rag.
Tape off the underside of the top and rub the legs with boiled linseed oil and a rag. Linseed oil is a cheap finish that brings out a warm glow while nourishing the wood. It is non-toxic and low-maintenance, easily refreshed with a new coat whenever necessary.