If you want to install custom ROMs and also backup/restore ROMs on your rooted Galaxy Nexus, you will need ClockworkMod Recovery installed on your Galaxy Nexus.
ClockworkMod Recovery is a tool that allows you to backup, restore, and install ROMs.
Before we begin, make sure you have rooted your Galaxy Nexus.
Step 1. Let’s turn off our Galaxy Nexus then boot into Fastboot mode by holding down Volume Down, Volume Up, and Power button together for about 10 seconds.