RepRap stands for Replicating Rapid Prototyper and is also the name of the group aiming to create an open source machine that can do just that. Find out more at RepRap.org. This is our (Me, Scott, and Joel) experience creating that machine.
Step 1: Project Planning
The very first thing we decided to do was create the frame. Wood is cheap and plentiful, and since we scavenged a bunch of loft wood from the dorms last we had tons. We needed to decide on how our design would be set up. After drawing up some pictures, and discussing it for a while, we decided to split the stages into two parts. The top of the machine would be the X stage which would hold the print head, and the bottom part would hold the YZ stage.
Our linear stages would be using smooth rod scavenged from a broken copy machine to hold the stages. We made the inner dimensions of our frame the same as our rods, although now I would suggest making them an inch or so smaller so you dont need to add extra boards to mount the rods on. As for height its not too important, but we used something like 3 feet.
Step 2: Build the Frame
First we cut four boards to length for the vertical posts, then we cut 8 boards for the sides. We essentially just created a box frame. Assemble the side boards into two rectangles, making sure that you keep them square. Next, attach the vertical boards on the inside of each rectangle. Keep making sure that your boards are square... this will really help things out later. Finally attach the second rectangle to the assembly, and you will be nearly done. Once we had it assembled, it was a bit wobbly, so we added diagonal cross bracers. After that it was rock solid and very sturdy.
Next, you will want to mount some boards where you want the YZ stage to go. Mount this about halfway up the frame, as they will protrude below the mount when the Z stage is all the way down. You can always adjust the height once you get the whole thing assembled.
If you want, you can optionally pimp out your lovely box with some wheels to help you move it around. These simply get screwed to the bottom of the posts and definitely give your RepRap some street cred.
Now you have the frame assembled, and its time to create the linear stages!