Large, useless, furniture is the worst to get rid of. Your significant other wants it out of the house, no one on Craigslist wants it, and it's expensive and hard, especially with two coupes, to take to the dump, not to mention wasteful and hard on the ol' environment. This was my predicament when we moved into our new house recently. The previous owners left an old, useless desk in one of the rooms. With a few parts I had laying around and very little time I up-cycled this collection of scrap wood into a useful mobile workbench that I can roll around my garage. No more walking back and forth from my project to the other side of the garage where I keep my tools.

Step 1: Materials & Tools

Materials

Desk - Free - If you don't have an extra desk or bench sitting around check craigslist, you probably can find one for free or very cheap. Thrift stores are another option.
Caster Wheels - ? - I had some casters in my scrap parts box, I'm sure your local hardware store carries them, even an office supply store might carry replacements for chairs.
2x2 or Scrap Wood - Free - I used pieces off of a spare 2x2 post from my deck. The size of the piece will depend on how your desk is constructed. At a minimum it will need to be thick enough that you can press the entire post of the caster into it and have the caster clear the bottom of the desk.
Wood Screws - Size/construction of desk will determine total number needed and size.

Tools

Drill
Drill Bit - The size of the bit should be just under the size of the post on the caster wheels. You won't need a really tight press because the weight of the desk will keep the wheels in but if your holes are too big the wheels will wobble and eventually break the support.
Driver Bit - Philips or Standard? Look at your screws.
Hammer - You might need this to press the casters into their holes.
Scarp boards or some sort of short platform - Used to get the desk back on it's wheels without breaking them off.

Step 2: Attach Supports

First of a warning that will save some of you some time and frustration: Unless your desk is made of a solid hard wood and you have a decent amount of edge clearance around where you want to but the wheels DO NOT skip this step and try to install the casters directly into the desk. I tried that and the wheels busted out with in the first few seconds of rolling. You can see the evidence of this failed attempt in some of the photos.

Flip the desk onto its side or top.
Measure and cut your support. The support should run from the front to the back of the desk. Make sure the support is resting flat against the bottom of the desk, and flush against the side edge.
Put at least two screws through the side of the desk into the middle of the support. Spread the screws out and make sure they are not passing through where you wanted to place the wheels. Keep adding screws until you are staisfied that the support is stable.

Step 3: Add Casters

Grab your drill and lock in your bit. Drill four holes, two in each support. Each hole should be on the center-line near the end. The holes should be deep enough that the caster posts go all the way in but not deep enough to punch through the bottom of the desk. Any depth between those two is acceptable.

Before pressing the casters in it's a good idea to clean up the holes, removing as much debris as possible.
With holes drilled and cleared begin pressing the casters in. If you are using swivel casters you don't have to worry about orientation, just push them in all the way, using a hammer to LIGHTLY tap them in if necessary. If you are using two fixed casters then I would suggest using a square to make sure the wheels are straight.

Step 4: Flipping the Desk

With the wheels firmly in place it's time to right the ship. You can't just flip it over by rocking it over on the front, or back, edge. Doing this would snap the wheels off. If your workbench is small and doesn't weigh much you can put it on it's side and just pick it up and set it on the wheels. My desk was much to big and unwieldy for this. If this is the case I have come up with a simple easy method for safely getting everything the right way up.

Lay the desk on its back, or front.
Lift the edge of the desk that is on the floor and slide in a stack of wood that is taller than the wheels.
Rock the desk up over the edge that you propped up, setting it gently on the front wheels. If you start rotating and the back wheels hit stop, go back, and add more boards.
Lift the edge that is propped up slightly and slide the boards out. Set it back down, now on all four wheels.

Step 5: Customize

Customize your bench based on what you will be using for. You could turn it into an electronics bench, woodworking bench, a general use workbench or any number of other specific purposes. I will be using this workbench primarily for working on and maintaining my vehicles so I set it up to hold tools only used for that (jack, jack stands, ect.), parts and supplies (replacement parts, oil, etc.), and other modifications to help while working. Post your thoughts for mods in the comments.

Current Modifications

A bracket, made from the left over support wood and another small piece of wood, that holds my creeper on the back of the bench.
A power strip, attached to the side of the bench, allows me to run multiple electronics and tools at the same time when it is plugged in.

Future Modifications