All about recovery
One important thing you must know about before venturing into modding is that in most cases, your Android phone's internal memory is partitioned in the following way: splash1 boot recovery system cache data What’s important here is the recovery partition, which contains the software relevant to the recovery mode your phone can boot into. The stock version allows you to reset your phone and reinstall the firmware. Now, in order to install most custom ROMs, we need to flash a custom version of our recovery manager, which allows us to do more than just reset our phone. One such popular software is the ClockWorkMod Recovery Manager, and installing it is as easy as installing the ROM manager app from the Android market. Then, to boot into this recovery mode, a special device- specific hardware key combination is used; you can easily look this up on the Internet. Most probably, the keys will be something like the ‘ volume up’ or ‘ volume down’ button that needs to be pressed while booting your phone.
A word about nAndroid backups is a must here. ClockWorkMod also provides an option for backup and recovery, among other features. This basically takes a complete backup of the memory and software in your phone, and copies it onto the SD card. So yes, you need enough free space there before attempting a nAndroid backup. Most developers suggest that you take a nAndroid backup every time you flash your phone.