One important note, this desk is not the most stable in terms of being able to take a good hit, so be careful or you might find yourself hop-scotching your laptop.
Step 1: Desk Parts and Pieces
2 - PVC pipe, about 4", with an inner diameter large enough to fit around the supporting cross brace of the treadmill.
3 - Rubber bumper feet of your choice, these are used to keep the laptop from sliding off the desk. Use as many as you feel you need and remember they will be rubbing against the laptop, so you'll want something non-marring.
4 - Wood for blocking the board away from the underside of the treadmill console, anything you've got that doesn't angle the desk so much the laptop wants to slide forward should do fine. The blocking just has to transfer the load of the desk closer to the middle meaty part of the treadmill console.
Of course whatever screws you have that won't go all the way through the table top of the desk, a compound or chop saw and a drill and/or drill press.
Step 2: Plastic Pipe Brackets
The two brackets should be in line so they will both line up with the cross piece on the treadmill. Just as important is to ensure the board is crowned or cupped upwards with these brackets on the bottom. If the board is crowned down the laptop will tend to rock side-to-side.
Also check that the brackets when installed still allow the desk to fit in the space between the cross piece an the treadmill console.
Step 3: Rubber Stops and Wooden Blocking
Add as many as you need, just make sure your laptop still fits.
The wood blocking is also attached to the top at the desk on the opposite end away from the rubber bumpers. Drilling and counter sinking these as well will help keep them from cracking and keep them from scratching up the underside of the treadmill console, if you care about such things.
I wanted the convenience of putting the desk on from the top of the treadmill, so I made sure the wood blocks also fit between the cross support and the treadmill console.
Step 4: Final Fit
The desk is front heavy and angled slightly down with the laptop on it so it sits stably while typing and walking... as long as you don't get too rough.