What you will need:
A backup of your Psion's disks. A backup is very important because everything in your C: drive will be erased. Your D: drive likely be safe, but a backup is still worthwhile.
The ARLO for your model of Psion.
A kernel image (and kernel modules if you are using those)
Option 1: A compressed initrd (a very limited system)
Option 2: A prepackaged distribution to be installed on a compactflash drive (Debian dpkg based - Sarge, most likely). Requires ca. 256 MB or larger CF card.
Probably not needed: A paperclip or similar, thin, blunt item (to reset your machine if it hangs)
ARLO, the kernel image, a compressed initrd and a prepackaged distribution can be obtained from the Arlo section of the www.openpsion.org File releases website (and elsewhere - see various links from the OpenLinux pages). ARLO, a kernel image and the initrd can often be found bundled together in a tarball.