Post [Q] installing a custom rom on a new released phone

hi all
at first sorry if i post my problem in a wrong place because in the Q&A thread i couldn't get my answer ( I've waited 3 days )
i have LG L90 D405 and i wanna have a custom rom on my phone
1) what should i do?? i mean how to unlock bootloader and root my phone and also flash a custom recovery and a custom rom??
i read some threads and i learned that i have to write some codes in a terminal ( im not sure what terminal they are speaking of ) also learned i must have cygwin and some other applications to unlock bootloader but the problem is that there is not a complete answer to do it step by step from first to the end
i appreciate if you help
thanks a lot