°Bootloader(It must be unlocked)
°Custom Recovery(Install it via root or fastboot)
°Your Diveces official firmware(In case of any problem) and
°Your Custom ROM Firmware
First check if your devices bootloader is officially unlockable or preunlocked(via fastboot). Then install a custom9 recovery(I will suggest TWRP). Now copy the ROMs firmware to your devices internal storage or external storage. Now boot your dvice to recovery mode. To boot your device into recovery mode first turn it off then press and hold the power and volume+ button at same time(The key combination may vary on your device. Just google it.) After booting in your custom recovery wipe system,data,cache,delvik-cache. Now select install and browse your ROM firmware and install it. And about 3-5 mins. After successful installation reboot your device and it should boot to your ROM. And the work is done.
A word,when you select a ROM be specific that it ment for your device or you will brick your device.
Before installing a Custom ROM backup your internal storage in terms of any file loss.
So now you know that how custom ROMs are installed. And if you enjoyed or learned be tuned to my posts.