Your favorite custom rom downloaded and saved on sd card root, or internal sd card root
Then you must go into recovery mode (power off phone, then press volume up + power).
After this, you must follow this steps(in square brackets optional steps):
Wipe data/factory reset ( row 4 -> row 1 )
Wipe system ( row 7 -> row 8 )
Wipe cache ( row 7 -> row 9 )
Wipe cust ( row 7 -> row 11)
[Wipe data] ( row 7 -> row 10)
[Wipe internal SD] ( row 7 -> row 7)
ALERT: wipe internal SD only if you have put your custom rom zip in external SD card
[Wipe exsternal_SD] ( row 7 -> row 12)
ALERT: wipe external SD only if you have put your custom rom zip in internal SD card
Flash your custom ROM if saved on internal SD card ( row 2 -> row 1 -> select ZIP)
Flash your custom ROM if saved on external SD card ( row 2 -> row 4 -> select ZIP)
Wait for installation
Back to main menu (there is only one row to select), reboot (row 1)