I did the y-axis and here is the x-axis
Step 1: Downloading and Printing
To start you need to download the parts from:
http://www.thingiverse.com/thing:194686 and print them out.
Step 2: Taking the Printrbot Apart
Once the parts are printed you have to disassemble the printer starting with the bed. You then take off both sides, bottom and top so that you free the motor and motor mount.
Step 3: Reassembling
You screw the new motor mount to the motor and then put the sides, top and bottom back on.
Step 4: Bearings and Belt
Once the motor mount is on you need to put on the bearings, spacers and gear just like you did when assembling the y-axis.
Step 5: Re-attaching the Bed
Before you reattach the bed you have to drill a hole on the opposite side for the x-limit switch. Then once this is done you re-attach the bed 180 degrees from its original position so that it clears the bearings and belts. After you can put the top piece of the bed on and add the belt supports.
Step 6: Tensioning the Belt
Once the bed is on and the belts are attached, you can add the belt tensioners which ensure that it is tight and securely fashioned. (Note: the bed in these last photos is a bigger custom one I made that will be shown in the next instructable)
All of this was done at the Tech Shop SF, you can visit their website at techshop.ws