Booting from CD is one of the easiest way to start and install your PA-RISC machine; assuming you have a CD drive handy and a bootable CD. You can download official Debian ISOs as well as Net Install ISO (see netinst) from the Debian Installer pages, or from the PA-RISC/Linux official website.
Start the box and enter the BOOT_ADMIN mode. (Section 2.1.1)
Place your bootable CD on the CD tray and close it. Sounds obvious, but we know guys who missed that step :)
There are two options from there: either you know the full PATH to your CD device, then you can jump to next step, or you don't. In this last case, issue a search ipl to list all available bootable devices. You can also specify search [PATH], which is fastest. For instance if you want to search the SCSI bus:
On recent boxes, search disk is quite helpful. Take a look at help search for details specific to your box.
Once you know the full PATH to your CD drive, you can issue a boot <PATH>. That's all. If everything goes fine, it will start booting the CD present in the CD reader. Real life example: