First: Why bother?! Yes, you can just buy a Fuji Instax Mini camera and fire away. But those don't give you manual controls, you work with slower lenses, and you have no control over the onboard flash. Using a Rolleiflex means full manual control, Zeiss glass, and access to any lighting setup you want via flash sync (if you want).
Things you will need:
-A pack of Instax Mini Film
-A Rolleiflex or other TLR.
-A dark place, or light tight film changing bag
-A set of rollers to run your print through
Steps to shooting instant awesomeness through your Rollei:
1) Place Rolleiflex and Pack of Instax film inside a film changing bag
2) Manually eject the dark slide from the Instax pack (use your fingers and gently guide it out the side)
3) Manually eject one exposure in the same way.
4) Open up the back of the Rollei, and place the instax exposure in the back, where film would normally sit to be exposed, then close up the back. Note - orientation is important. What you think of as the 'back' of an instax photo is the light sensitive part. So the 'back' of the print needs to face forward when inside the camera.
5) Take Rollei out of film changing bag, and expose as normal. Box speed of Instax Mini film is ISO 800.
6) Return Rollei to film changing bag, open back and remove Instax exposure.
7) Run the Instax exposure through rollers to squeeze chemicals out to start developing. This is a clumsy process, so I cheat - I reinsert the exposure into a pack of Instax film, and then eject the exposure through a Diana Instax Back (it pushes the exposure out through a set of rollers triggering the development of the print evenly). You can get away without this, but it will be harder.
8) Watch in awe as image magically appears before you eyes.
*In case it wasn't clear, all handling of the Instax film (loading, unloading, etc) MUST be done in the dark (film changing bag or a completely darkened room). This is a pain, and is not convenient. But INSTANT PHOTOS! FROM A ROLLEIFLEX TLR!
Above: Cactus Trigger, and Nikon SB800 (hidden behind me) bounced off the ceiling.
Below: No speed light. Bathroom incandescent lights were the sole light source.
If you still have questions, drop me a line in the comments.
Update: I googled and found everyone links to a page that shows a 'hack' for a Rolleiflex TLR with a tintype back. Note, there really is no need to hack your Rollei - you get the same result this way by simply placing the film inside the regular back of the camera. The only difference is that the tintype back will ensure your Instax frame will be consistently straight and upright (no tilt). You can get similar results simply by using some gaffers tape along the inside edges of your rollei to create a sort of guide rail that you would just butt your Instax frame's edge against before closing the back up.