Hi there!
Today I want to explain you how to install postmarketOS on N900...
The system will be running LXQt, light DE.

Requirements:
- Nokia N900
- Micro SDHC card, up to 32 GB (min 4 GB)
- PC with Linux (I use Arch Linux)
- Some time

WARNING
I am not responsible for any damage, lost data, etc.
All things you do - you do for your own responsibility. If you will break something - go to your mum, and cry

NOTES
OS will be installed on SD Card.
In this tutorial we install bootloader - U-Boot
eMMC will be safe.

Let's start!

1. Install pmbootstrap on your Linux distro on PC, via your way. On Arch, I used AUR to install -> https://aur.archlinux.org/packages/pmbootstrap
2. Open terminal on PC. Let's type few commands:
- it will create our workspace dir.
- it will ask us for work dir. Let's select the dir we just created:
~/pmos-n900
3. Now, it will clone repo. Let's be patient.
4. Now, it asks or channel. For best results, select edge
5. Now, few other options. I will just give right answers:
Vendor: nokia
Device codename: n900
Enable this package: y
Keymap: what you want. Depends on your favourite and you can also select that combines with hardware keymap.
Username: what you want
User interface: now, it's bit more difficult to choose. I would suggest you to select Xfce or LXQt.
Change them: n
Extra packages: what you want. I would suggest few useful tools, like nano. We write like in this example: firefox,nano,talkmaemo,n900
Use this timezone instead of GMT: Let's select y
Choose default locale: select your favourite.
Device hostname: select something short like "n900"
Build outdated packages during 'pmbootstrap install' - y
6. It will say done, now it's properly configured.
7. Insert your SD to PC. Let's check where it is via simple command:
8. In my case, it was simply: /dev/mmcblk0
9. Now, we install pmOS to SD! Command on PC:
10. It will take some time. Let's be patient. When the process ends - remove SD from PC and put it back to N900, and close back cover.
11. Boot to Maemo. Now we need to install U-Boot. In X-Terminal:
It will ask for flashing kernel - agree.
It may ask for flashing kernel - DISagree.
12. Now, we create item file for U-Boot. Let's create file:
13. Now type all lines very carefully:
Save it with Ctrl + O, Ctrl + X.
14. Run command on XTerm:
15. Reboot, select postmarketOS, done!!!!!!!!!

For more, ask here. Hope I will help someone I will try to make this post bigger, when I will have more time

Goodbye