If you have ubiboot installed then you're already halfway there. There is a new version ubiboot but it isn't required as Leste is booted without an initrd. The new ubiboot does however now include a nice Maemo Leste graphic and a few things preconfigured for it. That reminds me that I should send a pull request to juiceme to set a few more defaults for Leste (described below).

To get Leste running, you'll need to create a new partition (ext4 is fine). Then you can unpack the .tar.gz dist for the N9 to that partition and configure ubiboot.conf to use it. Assuming you'll use slot 5:

• Set G_OS5_INITSCRIPT to "\/sbin\/preinit" (those are "\" and "/", not Vs)
• Set G_OS5_NUM to "1"
• Set G_OS5_PARTITION to the partition number of your new partition
• Set G_OS5_NAME to "Maemo"
• Set G_OS5_1_LABEL to "Maemo Leste - Linux 4.16" or whatever description you like
• Set G_OS5_1_FILE to "/boot/Maemo/boot/zImage".

I'll write up a guide on the wiki once I've sent that patch to juiceme but in the meantime the above should get you going.

As I already mentioned, Hildon is broken on the N950 so I suspect it might be broken on the N9 too. So don't expect wonders, but USB networking works so that will give you complete access to the Devuan-like system to play around with.