Here's what you'll need:
3 60 lb bags of 5000 psi concrete.
2 Boxes of Cheng Concrete Countertop Pro-Formula Mix.
1 3/4" sheet of plywood.
2 8' 2x2 or 2x4.
2 Pieces of scrap lumber to make molding radius.
1 8'x2-1/4" silicone concrete edge molding.
1 3'x4' dry-erase white board.
2 Sturdy sawhorses.
12 2" x 5/16" bolts.
4 1/2" x 27" hairpin legs.
1 2-1/4" piece of 1-1/2" pvc.
Step 1: Come Up With a Plan.
For the edge I decided to go for a split-face rock surface. I bought the silicone molding on eBay. I wanted the table to be about 28"x44", so an 8'x2-1/4" piece was perfect, since the back face of the desk would face the wall and needed to be flat. For that I used a 2-1/4" piece of the same dry-erase board cut to length.
Step 2: Create the Form.
I used a brad nailer to attach the molding to the form. It worked well, but there were a few brads that I had to drive deeper with a punch. Be safe using a brad nailer. I'd recommend gloves and obviously safety glasses are a must.
An option is to put a cord hole near the back. I don't have a picture of it but I took a piece of 1-1/2" pvc and pushed it in place once the concrete was placed. You can see the end result in the first picture in step 4 or the second picture in step 5.
Step 3: Place the Concrete.
Load the concrete in and start vibrating it to get the bubbles out. You could use a vibrating sander on the bottom of the form, but I didn't think it would be enough. I used a heavy deadblow hammer on the bottom of the form. It worked well. I wouldn't use a vibrator in the concrete, as you would likely scratch the form surface, which will translate directly to the finished surface. Once you are fairly confident that you've vibrated out the bubbles, place your reinforcement at the top of the concrete when you have the form almost all the way full. You don't want to put this in place before you vibrate the concrete, since it will naturally sink down and get very near the eventual top surface. That's not where you want your reinforcing tensile strength, of course. It needs to be near the bottom of the desktop. I'd shoot for about a half to 3/4" of cover.
As for reinforcement, I used 6x6 welded steel wire. Since the desktop is only 2-1/4" thick rebar wasn't a great option, though I'm sure you could do it. One last step is to set your bolts for the table legs. I decided to use a vintage style hairpin leg, so I made a wood form to set the bolts and sunk them once the concrete was all in place and the reinforcement was placed. After that, walk away. Stay away. I know concrete sets up quickly and you should be at 50% strength in 24 hours. I get that. Stay away. Let it strengthen in it's form for a few days or even a week. I promise you won't regret that. And the forms will come off no problem. Seriously, leave it alone.
Incidentally, I know that second picture isn't good. It's wet concrete, you can't expect much.
Step 4: Strip the Forms.
1. It's heavy. Don't mess with it by yourself.
2. It's heavy. I'm not kidding.
I did something dumb and potentially unsafe. Focus not on what I did, but on what you should do. Have a buddy help you with this, or maybe two. The desktop is probably about 200 lbs, so two people can handle it, but it's nice to have someone else help you steady it as you flip it or move the sawhorses, etc.
Once you unscrew the 2x2 forms, you can pull the silicone molding off pretty easily. In fact, if you used the brad nailer like I did, the molding will come off with the 2x2. It's nice to see the edge, but the real money is in the surface. All this work is for naught if it's not pretty and smooth.
Not to worry, smooth as glass. Once you have stripped the form, securely store the slab in an upright position and use a water sprayer at least a few times a day to keep the surfaces wet. It's kinda cool to see the water bead up on the finished top surface, frankly.
Step 5: Install the Legs.
That's it, you're done. Congratulations. You've created a concrete computer desk with a high sheen, all without having to actually "polish" any concrete. Way to go. I'd love to have your comments, suggestions, or questions. Thanks for looking.