OK, please read the dislaimer on the install page (Install WOP for Tenderloin - WebOS-Ports) carefully as these instructions are from memory:
I had webOS 3.0.5 and CM9 installed on my touchpad and this is what I remember doing to install LuneOS, creating a triple boot with the existing OSs:
On the wiki, perform step 1 (Create an ext3fs lvm partition). I used Tailor.
I installed tailor and rebooted, but even after reboot I kept getting an error "Danger Will Robinson... Jails are active...". I found a post from Rod when someone had that issue and he asked what mounts were present, the reply was a mount related to tweaks and Rod said to unmount that and continue. This did the job for me. Just needed to "Check File System", "Resize" the usb partition to reduce it by about a gig, then "Create new partition" in the unused space.
You may be able to skip steps 2 and 3 of the install instructions if you already have Moboot installed and don't want to update it. I had Moboot (0.3.4 from the CM9 install) and initially successfully booted LuneOS, but later decided to update Moboot to the version recommended in the wiki.
Peform step 4 to download the uImage for LuneOS.
I used the command line from webOS QuickInstall rather than Novaterm only because I was more familiar with that.
Perform the first line of the commands:
mount -o remount,rw /boot
You can skip the "rm uImage" if not updating Moboot
Perform the next command line to put the uImage file as uImage.LuneOS (again I used the webOS QuickInstall file transfer)
If you are updating Moboot, perform the next line to put the uImage.moboot_0.3.8 as uImage
You can skip the line that starts with "ln -sf ..." as this is already done from the CM9 install.
Perform step 5 on the wiki. I had to unmount (eject from Windows file explorer) the touchpad's usb partition as it was mounted from copying the file over.(note that you can download the non "dev" image, just make sure you adjust the "tar xzf ..." command with the filename you downloaded)
Hopefully I haven't left anything out or made a mistake that causes you to kill your existing installs.