After rooting your Android phone, flashing is just a piece of cake.
- Put your new android installation file in SD Card (it will be in ZIP file)
- Boot your phone in Recover Tool
- Do a Backup, just in case (it will backup everything inside your internal memory)
- Wipe your phone internal memory (normally MUST if you are flashing a completely different Android, but if it is upgrade or downgrade of the same Android, probably can skip)
- Then install zip file from SD Card
- Reboot and done
The only painful thing of flashing is that you might need to reconfigure everything and install back what application you want (just like format a PC). So the more tricky part is to pick the correct one. You can only know this version of Android is your favorite or not until you use it, and switching between different Android is troublesome.
The customized Android is roughly divided by 2 types: Android with HTC Sense UI, and Android with AOSP UI. After choosing the UI that you want, then choose the version based on the application that come with it.
Some major player of customized Android include cyanogenmod, defrost, oxygen and some more.