I needed to create a couple of computer desks for a home office, but with off-the-shelf computer desks being hideously expensive for what you get, the current prices and availability of plywood being an issue, I wanted to do it whilst minimising waste. In typical fashion I built these around 6 months ago and didn't plan on an instructable, let alone entering the One Board competition. (facepalm!)

Using Sketchup, I calculated I only had enough ply left for a 30cm x 30cm sq board, and if my terrible math is roughly right, that's a 97% usage rate for the board.

Supplies

1 Sheet of 18mm ply with a good quality surface (ie, 'furniture' grade). Construction ply will work, and is fine of you want an industrial look to your desk or you are going to paint it. Just be wary of internal gaps as they can really ruin the edge if you hit one.
Wood glue...lots of!
Clamps..lots of
Lengths of timber to help spread the clamping pressure across the panels.
Fixings to assemble the desk components. There are various options here:
Glue + Screw them together,
Glue + dowels
Pocket holes (personally I'm not a fan of that one)
M6 MDF inserts and bolts/threaded rods/washers + nuts - This is my method as the only screws you can see are the ones that connect the legs to the back panel. And these are the fancy hex bolts. I can also disassemble the desk for flat-pack transport.
Circular saw with a good sharp blade,
Belt sander and/or orbital sander with a range of grits
Finishing coat (paint or polyurethane)

Step 1: Measure 3 Times, Mark, Re-measure, Cut

This is the sheet plan I made in Sketchup to mock up the cuts and the assembled desk. The legs are made up of 2 single pieces with offcuts laminated between. This gives me clean sides and a total thickness of 54mm.

Note that this does not take into account the thickness of your saw blade, so run the blade down the middle of each line to get 'near-enough' parts.

Also, a word of warning - Some cuts require starting with a plunge cut using the saw. Do it with care, slowly and a firm grip on the saw, and make sure the area behind the saw blade is clear of things including body parts. Once you start the cut, don't attempt to correct a misalignment for risk of a kickback. Hold the saw firmly then release the trigger. Wait for the blade to stop spinning, lift the saw back out, re-align and start a new cut.

Some cuts will have to be finished with a handsaw as you won't be able to do a full cut into the tight corners.

Step 2: Glue, Clamp, Wait and Sand

As seen in the exploded view, glue and clamp the legs and top using the parts. I have colour coded the parts of one leg to show how the sections are layered to hide the joins between the clean sides.

Key points to note:

Keep an eye on the alignment of the leg layers. I found the middle parts tended to move as I applied more clamping pressure, like they were being squeezed out.
Make sure the table top stays absolutely flat until the glue dries. This is going to become a 36mm thick slab and any twists will not be able to be corrected, leaving you with wobbly legs..on the table too.
When sanding, remember you are sanding the edges and can cause a belt sander to grab and throw you/the wood around. Best to let the weight of the sander do the downward pressure.
Don't be tempted to use heavy sanding on the surfaces. Some plys have a very thin clean layer (cheaping out by using the bare minimum layer thickness) and it does not take much to rip right through it.
If using polyurethane to finish the parts, use a thinned out first coat just to seal it, then apply 2-4 layers as normal on the top. I finished mine with 3 top coats and letting the last coat fully dry, an orbital sander with superfine pads, then a polishing pad and car polish/wax.

Step 3: Assemble

Join the legs together with the back panel. It only needs 2 screws each side, and set the panel in from the back edge about 100mm. You can add hooks or a shelf behind it as a way to tidy up cables and power supplies, hidden behind the back panel.

Hint: The panel is only 18mm thick, so drill pilot holes before putting in screws, and if you can apply a clamp on either side of the screw holes, that will greatly reduce the chance of the ply delaminating. If using MDF bolt inserts, clamping is a "must do" because of the outer diameter of the insert. It does not leave a lot of wood between the hole and the outer layer.

Set the top, upside-down on your workspace (put a dropsheet or some other protective cover down first), then place the the legs on top and adjust so it is even and square all around. Attach the top to the legs using your chosen method. I used inserts on the underside of the top, and threaded rod + washers + nuts to bolt the legs to the top.

Hint: Use a long small drill to pilot hole the bolts and inserts to ensure good alignment, and use a spade bit to create a small recess to hide the nuts and washers prior to drilling the ~6mm bolt hole.

Step 4: Sit and Admire

This table is 600mm from floor to underside of the top and some might find that too low. You can add height by screwing some nicely sanded and finished blocks under each end of the legs. Alternatively you could use a second sheet and just scale up the leg sections to the preferred height.

But then again, it wouldn't be a '1 sheet' challenge now :D