First thing's first, you need to get an ARM version of whatever distro you want to run. If there is no pre compiled version, try downloading the source to your phone and compiling it on the device.

Next, you need to copy the image. For a first attempt, I'd say do so on the Memory Card.

Lastly, on the eMMC (internal storage), install whatever you need for multiple self-booting OSs to run and be switched between (at boot time). I suggest using external storage for the non-Maemo OS itself, in case it doesn't work.

You should be able to install it (your distro) on to the N900 at this point. This also where things are likely to go wrong. Hopefully the image has a self-installer that picks up on what it needs to, in order to both start and fully install.

As you might be able to tell, I haven't done this myself so the above comes with no warranties!

From a high-level, that is your how-to. Apologies I can't be more precise on a low (instruction by instruction) level. Check info on how to install Ubuntu - should give you a good idea.

... post back with how you get on