I will explain a little installation using MultiROM. It is best to drop rootfs.img and multirom_v2_n7-signed.zip to the root of the memory card immediately, so that you do not sweat and do it all at once. After that, we enter the device into fastboot, we download the modified recovery file into the folder with fastboot.exe (I have c: \ android \ tools), for simplicity, rename it to “recovery.zip”, open cmd and command:
cd c: \ Android \ tools
fastboot erase recovery
fastboot flash recovery recovery.zip
After that, we boot into the recovery, then install multirom_v2_n7-signed.zip from the memory card, just in case reboot the recovery, then go to the section Advanced ->MultiROM ->Add ROM, select Ubuntu at the top, search for our bootfs.img and click on it, then swipe and ... voila! Now when downloading, there will be a choice between Android and Ubuntu.
P.S. I hope that I described everything correctly, in a hurry. I would be glad if someone helped.