Wait.. Instax fits / works in an SX70 Heather? Any mods necessary to do! (Beautiful!)
It's an awful awful embarrassing faff. Here's the really long directions I used
http://www.flickr.com/photos/polapix/2328453771/Basically... you need an empty cartridge for the sx-70 at a minimum. I'm using my instax to store the unexposed ones and being very careful reloading it.
Put a bit of thick tape over the end of the sx-70/600/Impossible cartridge so the sx-70 can't eject anymore.
get everything in your darkbag/darkroom. Take one unexposed sheet out of the instax cartridge... stuff it in the sx-70 cartridge... load it in the camera. Put away the rest of the instax safely in the dark. Take the sx-70 loaded up and shove it in the sx-70 (it'll whirr like it's trying to get rid of a darkslide. it shouldn't eject anything though or else you'll end up with a mess).
Take picture.
put everything back in the bag, take the cartridge out of the sx-70, take the instax out of the sx-70 cartridge, put it back in the instax cartridge, then put it in the instax camera. you can take everything out of the bag now. Turn on camera, stuff the front of the camera into your leg/sofa/anything to block out light to rule out double exposure (unless you like that), then it'll pop out and develop.
It's really longwinded but once you've done it once (then messed up then realised what you did wrong) it's not so bad.
And it's such pretty colours.
Oh and the pictures turn out reversed/backwards. Doesn't really matter for the plant picture. Will matter if you have any text.