I've gotten really into the idea of having a standing desk lately (due mostly to lower back pain caused by poor posture), but I have a desk that I really love and didn't want to do anything permanent to mess it up. My solution was to build a platform of sorts that my desk can stand on top of.
Step 1: Drawing/math
First, I needed to do some planning. I determined that I wanted a desk to be 41 inches tall (through some trial and error - I stacked books under my laptop until it felt comfortable, then measured the books). My current desk (the Parsons Desk with Drawers from West Elm) is 30". So I needed a platform to be 11" tall. I also decided to make the platform a little wider than my desk, to account for any slipping. Measure your desk, and add an inch to each dimension.
Originally, I had planned to make two seperate platforms for the left and right sides of the desk. I realized pretty quickly that they'd tip over way to easily, so I stuck with that design and added a connector at the back of the desk.
Here's a rough drawing of each side platform (not to scale, and apologies for the blur - your numbers would be different from mine anyways):
Step 2: Supplies
Next, buying supplies. I planned on using sturdy 2x6 for each side platform, and 1x3 for the connector. I totaled up how much of each I needed (14' of 2x6, and 101" of 1x3), and how many pieces of what length I needed (8 - 2x6x8", 4 - 2x6x26", 2 - 1x3x50.5"), and set out to the hardware store.
Regrettably, I don't really have a workshop, so I had to ask the hardware store employees to cut my wood for me - they weren't too happy. While there, I also bought sandpaper to smooth down the cut edges (I'm not too concerned about things I build looking pretty - if you are, make sure to buy sandpaper in a range of coarsenesses).
Upon getting the materials home, I realized I forgot to buy screws, so I went back out. Since I used 2x6 for the majority of the construction, I needed to use screws long enough to go through 1.5" and then some (note - if you are not already aware, nominal dimensions for lumber are NOT the actual dimensions - the 2 inch side of a 2x6 is actually about 1.5"). The shorter screws were for attaching the 1x3 connectors.
Step 3: Assembly
Next, assembly. This was pretty straightforward - attach the 8" pieces to one 26" piece; attach the other 26" piece on the other side. After building the two sides, attach the crosspieces - now's a good time to decide which way goes up and all that.
Here's the lumber right after assembly began. I would've taken more pictures, but this should really be pretty obvious - just follow your drawing.
Step 4: Finished!
Here is the finished product, not under my desk. The two wide "wooden cinderblocks" go on the left and right sides of the desk, and are a fair bit wider than the legs of the desk. To move the desk onto this, I convinced my girlfriend to help lift it up (with laptop and everything else still on it) and move it over. There is a little wobble, but nothing at all worrisome - it's surprisingly very stable feeling. And really comfortable!