Android Flashing Guide

Prerequisites

To Flash a ROM for your Android device you first need to meet the following prerequisites:

1. An Android smartphone that has a custom recovery installed (Amon_RA, ClockworkMod, Etc.)
2. A custom ROM that was developed or ported for your device.
3. A MicroSD Card to hold the ROM file temporarily.

ROM Flashing

1. Obtain a ROM of your choice, many can be found on XDA Forums or an alternate android development site.
2. Once you have downloaded the ROM File, mount your phones SD card and and copy over the .zip file.
3. Power off the device and then boot up to recovery, it is different per device. 
4. In your phone's custom recovery bootloader, locate the menu options which allow you to wipe the phone. Whenever changing roms it is a good idea to make a Nandroid or BART backup first, then wipe Data/Factory and Dalvik-Cache before flashing the new ROM.
5. Once your phone has been wiped go back up to the root of the menu and select 'Flash Zip from SD Card' the text may be different per Recovery.
6. Locate the ROM .Zip file you wish to flash.
7. Confirm the flash by following the on-screen instructions (ie: pressing the trackball, pressing the call button, etc.)
8. If no errors arise during the flashing process then the ROM flash was successful, Reboot the device.

App Installs/ROM Feature Installs via Flashing

Some Roms have added features that are available to flash.

1. To Flash the App/Rom Feature you must first download the .zip file.
2. Mount your devices SD card and copy the .zip file over.
3. Boot up to your devices custom recovery.
4. Wipe your Dalvik-Cache in the Wipe Menu. And then go back to the recovery's main menu.
5. This step may vary per device, there should be an option that says "Flash .zip from SD Card", select it.
6. Select the .zip file you wish to flash.
7. Confirm flashing of .zip file.
8. Wait for the file to flash; and if successful, reboot.