It's not very easy and not much documentation exists.
Searching forums is your friend (I managed to find many useful advises on aw.net) and I
successfully managed to install Debian Sarge into my former A1200D PPC (you can check video
here)
Sadly even finding the correct info, you'll realize that some of the repos don't exist nowadays or they changed servers that you have to manually find
It's really fun as experimental procedure, but certainly is not fun to run. Just for example the installation took me a whole weekend with my Amiga on lol (was socked it haven't crashed at all).
Maybe by using command line mode only would be okeish, but X with Gnome... was painful as hell (even on my 330MHz BPPC).
Imho, best Linux distro for our Amigas is NetBSD for those who like to experiment and it's still supported and updated
