I have ubuntu installed on my computer and followed instructions but after I start ubuntu-device-flash it always fails at recovery flash. I tried using both stable and rc-proposed channel but it is the same every time.
I get a "cant flash recovery image" and on the phone there is no error mentioned. Last message displayed is fastboot flash recovery....something.
Before trying all this I had rooted stock Lollipop rom installed with twrp.
You already unlocked bootloader, right? Could you please post full message from both terminal and phone? It contains useful info.
That said, I've heard that Kitkat-based custom ROM (including mine) doesn't work with Lolipop-based bootloader. So, you may want to try flashing bootstack (http://forum.xda-developers.com/lg-l90/development/bootstack-kk-l-l90-t3118632) in TWRP. Make sure you know exact phone model number (Dxxx__). Then, follow the instruction again, while still in TWRP.