When you have backed up your phone then follow those steps one by one
1. Create a new folder on your desktop and rename it as per your choice.
2. Now move the downloaded “Android Froyo Custom ROM” to the renamed folder.
3. The Custom ROM is a RAR archive so you need to extract it using WinRAR.
4. Now connect your Sony Ericsson XPERIA X8 Phone with your PC via USB cable.
5. Select “Open Folder to view files” in the Autoplay box and then click on OK.
6. Now Go To backup folder present inside the xrecovery folder.
7. Copy “Android Froyo Custom ROM’s” folder to backup folder from renamed folder.
8. When it’s done, disconnect your phone from your PC.
9. Switch Power off your phone after a while switch power on again
10. and as soon as the Sony Ericsson logo flashes on the screen start pressing the back key continuously.
11. This will take you to the Xrecovery menu.
12. Now Select “Backup and Restore” in the Xrecovery menu click on restore( there is 2 files the Backup and the "Android Froyo Custom ROM” file ) select this last one
13. It will Ask “Are You Sure”. Select “Yes – Restore”
14. Now it will start installing “Android Froyo” on your XPERIA X8. Be patient, it will take some amount of time.
15. After five or six minutes installation gets completed and a menu comes in front of you. Select “Reboot” From That Menu and Press Enter.
16. Your Phone will reboot and you will see the change.
and voila Android Froyo get’s installed on your phone. Enjoy