Now, let me be clear, while these are adjustable jack stands, they are not really something that you'd want to change on the fly throughout the course of the day, so be sure, this is no substitute for someone who wants both a sitting AND standing desk. For that, you've got to shell out the big bucks and buy a motorized system. This diy solution is more for someone who wants to commit to a standing desk, but wants the ability to dial in the perfect height for comfort and ergonomics.
For anyone who is thinking about converting to a standing desk - take a tip from someone who already has, you want to raise your whole desk up, not just your monitor and keyboard. Sure it's easier to simply raise your monitor on a stand or arm and buy a small platform that your keyboard and mouse can rest on at the proper height, as many diy standing desk conversions online suggest, but then you lose access to all the good stuff on your desk besides your computer. Where do you put your coffee, paperclips, obscene but thoughtful hand drawn christmas cards from co-workers and the 800 cords for your iPhone, camera, SD card reader and external hard drive? Having the whole desk at standing desk height sacrifices nothing (except your chair), gives you easier access to all your stuff.
Step 1: Purchase Jack Stands, 4 Nuts and 4 Washers
Then, go to the hardware store and pick up (4) 3/4" standard zinc plated nuts, and (4) large zinc plated flange washers.
Step 2: Assemble
To accomplish this, first, slid the threaded rod that comes with the jack stand onto the base. Then, place the flange washer over the bottom of the threaded rod and follow it with the second nut.
Step 3: Slide Under Desk and Adjust to Proper Ergonomic Height
You can get a vague idea of how high you'll need to jack up your desk by using a simple standing desk height calculator here. That way, you can adjust the jacks most of the way before you put them under your desk, so you only need to do some fine-tuning once they're in place. It's a whole lot faster to feed several inches of threaded rod through the nuts before you plop your desk on top of them then once the heavy desk is in place.
Fine tune the height using the leveling feet and then threaded rods.
Even though this is really beyond the scope of this Instructable, my monitors had to be raised to the proper height as well. I bit the bullet and went to Ikea for some steel feet and a black wooden shelf to accomplish that. I'll publish something about the adjustable height monitor shelf that I'm working on now soon in a separate Instructable.
Step 4: Level
I used the original leveling feet on the desk to dial in the level, but it's really six of one or half a dozen of another whether you choose to turn the jack screws or the leveling feet.
Step 5: Secure
Screw an L bracket onto the bottom of the desk and into the wall and it will really help lock everything into place. Don't want to screw into your wall because it's made of brick and you are scheduled to be moving to a new office soon? Use a wooden shim slid between the desk and the wall to put some tension on the system and hold everything in place. I'm not too proud of this solution, but until the move, it works just as well as a screwed in braket to reduce any shaking. Earthquake proofing...not so much.
With this configuration of jack stands and the wooden wedge, my desk doesn't move at all, can be adjusted to the exact proper height so that my arm forms a right angle at my elbow, and can easily be changed should I come into work wearing high heels one day, but flip flows the next.