[HELP] How to compile Android stock ROM from source code?

Hello guys, I updated my phone to 5.0.2 and now I need to downgrade back to 4.4.4 urgently. I have downloaded source code of ROM exactly for model at Lenovo support site, however, I don't know how to compile it so it will be flashable from internal memory. I'm using Ubuntu 14.04 LTS.
Please help!