The following information is not specific to any Android device. See the links just above for device-specific info.
Android universal instructions (custom roms etc)
See device-specific pages for how to get into recovery mode, fastboot mode, etc etc
(not the android, a linux or windows or something)
Just download the zip, unpack, and put the binaries on your PATH or something.
(Pre-req to install custom ROM)
Download from https://twrp.me/Devices/
Install adb and fastboot on a computer
Enable USB debugging on the android device
To install TWRP:
Boot into bootloader mode (e.g. fastboot mode)
“fastboot flash recovery twrp-2.8.x.x-xxx.img”
ALTERNATIVELY if device is already rooted, you can install and use TWRP manager
THIS WIPES ALL YOUR DATA
Copy zipfiles to phone file system
Boot into recovery mode
select “wipe data factory reset”, “wipe cache partition” and “wipe dalvick cache”.
Choose “install zip from SD card” and “choose zip from SD card”. Pick the update file and flash the same.
Optional: repeat this operation for applying the Google Apps package.
(as opposed to the whole ROM)
Plug in the phone and boot to fastboot mode (back+power or camera+power). Wait until the screen with skating ‘droids appears,
press the back button (the center bar should say “Fastboot” or “Fastboot USB”)
On the computer:
fastboot flash boot boot.img fastboot reboot