The desk top was going to be too large to cut from a single sheet, and even if it wasn't, a single sheet was too large to get home in the car. So I had the timber shop cut one sheet into two long thin rectangles, and planned to join them, together with a further small triangle for the back corner.
I was concerned that marking out the lines for cutting out such a large shape would be prone to cumulative errors - a slight error in a right-angle at a corner would magnify along the lines spanning from the corner. So I took measurements from multiple pairs of corners from the plan in Sketchup (the tape-measure tool makes this very easy), and used several measurements to "triangulate" the position of each corner point.
As with the rounded-rectangle holes in the backs of the units (see my previous post), I cut the desk top edges roughly with the jigsaw, but I waited to do the second pass with the router once I had the three pieces joined together, to make sure the straight edges were neat across the joints.
Holes for dowels to join the three pieces were drilled very quickly thanks to the dowel jig, but glueing and joining the pieces was unexpectedly difficult - I could not use my sash-clamps to compress the pieces together because a) they're too small, and b) there's no straight edge at the back corner of the desk top to clamp onto. By the time I had realised that the pieces weren't simply going to push together (8 dowels along the longer joint put up quite a resistance...) I already had the glue on and it was setting. I pushed and hammered jumped up and down on it (carefully - dowels snap easily if a joint is bent) and eventually gave up with a tiny gap along the joint.
The end result is that the desk top has wood filler along the joints, which is not great, but you can only see it if you look directly above the joint. I'm looking on it as part of the character of my home-made desk.
Once the glue had set, I took the desk top outside (too big to fit in the garage!) and used the router to cut the final clean edge to each side.
Now it's starting to look like a desk!
In this picture you can also see that I've attached the drawer sliders and put the drawer in the left-hand unit.
I actually sat at the desk in this state and did some work for a few days. That felt so good that it spurred me on to finish it properly.