I was inspired when I first read the Instructables' staff's guide to building treadmill desks. It was leg pain from sitting too long during an intense work project, though, that finally got me to build my own.
After looking at pictures online of a number of other people's projects, I identified a couple must-haves for my own:
-Freestanding : I was planning to move, didn't want to make holes in the wall that I'd have to turn around and patch, and I couldn't be sure my next place would allow wall-mounting.
-Collapsible : I was sharing a one-bedroom with my partner, and we wanted the treadmill belt to be able to fold up when I wasn't using it to free up the space.

Step 1: Get a Treadmill

Following the advice of other builders, I started my search with a post in the "wanted" section of my local Craigslist.
Through lucky timing, I connected with a collapsible treadmill owner who in the process of decluttering.
Not all treadmills are collapsible, so I specified in my ad that I wanted a collapsible one, and verified this detail with people who responded to my ad before going any further. Before picking up the treadmill I was ready to accept, I also asked the owner to send measurements to verify it would fit in the small space available to me.

Step 2: Measure, Calculate, and Plan the Freestanding Shelf

I wanted everything to be freestanding rather than wall mounted in case I moved. I planned two parts to my desk:
1. A freestanding shelf that would fit around the front of the treadmill, with a top shelf at the height I wanted for the base of the monitor.
2. A separate keyboard/mouse shelf, to be mounted on the handrails of the treadmill.

To plan out the freestanding shelf, I first measured the width and height of the front of the treadmill at its widest and highest points, then used these dimensions as the minimum internal dimensions for the shelf.
Next, I calculated where the base of the monitor should be. I started with the ergonomic guideline that the top of the viewable area of the monitor should be even with my eyes. I stood on the treadmill and had a friend mark the wall even with my eyes, and then I measured the height of that mark.
Next, I measured from the top of my monitor to its base, and subtracted that from the measurement. The result was the distance from the floor to the top of the freestanding shelf.

I then calculated the dimensions of the shelf:
Depth: I used the depth of the plywood shelves I'd cut in Step 3 as the depth of the shelf itself.
Height: Based on the measurement from Step 2, I subtracted the thickness of the plywood top shelf, to determine the length of of the four leg pieces.
Width: I added 1 inch to the measured width of the treadmill to provide a margin on both sides, then added the width of the left and right legs, to arrive at the final width of the shelf.

Step 3: Collect Parts

I bought two 1x8 planks for shelves, four 1x1 boards for the four corners of the shelf, and a 1x2 board to serve as stabilizing legs. I also found a piece of thick ply wood that was big enough to make two shelves out of, and cut it down to the same width as the planks.
I also bought 50 #8 x 2" phillips flat wood screws and two #24 hose clamps.

Step 4: Cut Wood

I cut the two plywood shelves to the width of the shelf.
I cut the four legs to the height calculated in Step 2.
Using the remaining 1x1 scraps, I also cut four pieces to the same length as the depth of the shelf pieces, to be used as cleats for attaching the shelves to the legs.
I also cut one of the planks to the full width of the shelf, to be used as a cross-brace on the back.
Finally, I cut two 3-foot lengths of 1x2 to serve as stabilizers.

Step 5: Assemble Shelf

I screwed the cleats to the legs at the top, and then attached the top shelf to the cleats.
I attached the cleats to the legs where the lower shelf should go, and then attached the plan to the back of the shelf to serve as a backboard for the bottom shelf, while stabilizing the shelf to avoid left/right ricketyness.
I attached the two stabilizers to the legs.
I then used a jigsaw to cut notches from the four corners of the plywood for the lower shelf and dropped it into place, supported by the cleats and the four legs.
When the shelf was assembled, I put it into place and moved the treadmill inside it.

Step 6: Measure, Cut, and Assemble Keyboard Shelf

I found many plans online for treadmill desks with keyboard trays attached permanently. However, I wanted to preserve the collapsibility of my treadmill in my small apartment, so I had to be able to move the keyboard tray out of the way when stowing the the deck.
I decided to use the metal safety bars on both sides of the treadmill to mount the keyboard tray. However, I decided to create a tray that could rest on the bars without being permanently attached, letting gravity hold it in place and allowing me to easily remove it when necessary.
As with the top shelf height, I started with the ergonomic guideline that my forearms should be parallel to the floor when typing. I stood on the treadmill with my arms extended and had a friend mark the height of my forearms on one of the metal treadmill railings. I then took the second plank and let it rest on the railing so the mark on the railing was even with the top of the board. Using a level on the plan to make sure I was holding it parallel to the ground, I measured from the bottom front of the plank to the metal railing below it. I would need to create a support to make up this height.
I cut a scrap of plywood to the width of the plank, then drew a line marking the final height I needed to brace the plank. I then held it over the treadmill railings and marked the center of each. I then cut semicircles out of the wood with the edge of each circle centered on one of the railing locations, and even with the line I'd drawn.
I then attached the plywood scrap to one edge of the plank, driving screws at an angle to avoid showing from the front.
Next, I attached a hose clamp to each of the treadmill railings, approximately in the location where the front support piece should rest on the railings. I left the hose clamps loose to allow for variances in the semicircles and in the attachment of the support and the plank. I then put the plan in place, with the semicircular holes resting on the railings just above the hose clamps; the hose clamps keep the keyboard tray from falling off.
Finally, I put a level on the keyboard tray, adjusted the position of one or both hose clamps as necessary to make the tray level, and then tightened the hose clamps as tight as possible.