It's pretty simple really, but see the step-by-step instructions at Instructables.
Basically, you do this:
- Find two cheap bikes you don't mind sacrificing.
- Secure the front fork by drilling holes through the headtube and sticking bolts through so the steerer doesn't move.
- Cold set the rear bike's front forks by spreading them out so they fit over the front bike's rear forks/dropouts, then put the "middle" wheel in and use the rear wheel axle to hold the whole thing together.
- Run a drive-side timing chain from the front bike to rear bike's crankset. The front derailleur is removed, of course, as are both derailleur's from the captain's bike.
- The drive chain is run from the rear bike's other chainring to the rear bike's rear derailleur.
- Cable up shifters and brakes.
- Ride it and have fun.
Details at Instructables.