What you need to prepare before installing/flashing custom ROM/Firmware on your devices.
1. Need to know the model number/name/brand.
2. Look for any custom rom/firmware build for your devices out there by GOOGLE-ing it or search at XDA forum.
3. Read the 1st post by the ROM DEVELOPER, because they always put some reminder for what is working and what is not.
ONLY CONTINUE ONCE U FULLY UNDERSTAND THE RISK OF FLASHING CUSTOM ROM/FIRMWARE
4. Install all the necessary driver at your PC/MAC.
5. U must have a ROOT ACCESS for your devices, how to root your devices? again...GOOGLE it :)
( superoneclick,z4 root for older devices (gingerbread), and other root application)
6. Must have custom recovery, famous one is CLOCK WORK MODE (CWM) recovery, again, u can always GOOGLE it or find it at XDA forum thread.
7. If u choose to flash:
- an AOSP based rom, u must have 2 files for flashing (the rom file XXXX.zip and Gapps.zip)
- stock rom (released by phone manufacturer and cooked by dev) just one file, but super large file, usually 200mb and above.
8. Put the rom files in your sdcard and flash it from CWM recovery, and u are DONE.
9. Congratulations!!!!!your devices warranty are now VOIDED!welcome..hehehe ^__^
10. U can always install back stock rom (untouch) to claim for warranty :) no worries
do drop me a comment or an e-mail for help :)
I'll try my best to help