you need to replace/install another "recovery" than the original one on you phone first
with SP Flash Tool (PC application).
With this recovery it is possible to create Nandroid Backup (do it first before flashing other ROM)
Copy a ROM in the root of sdcard, in recovery mode chose your new ROM in zip-format you want to flash,
or reflash Backup you have make (sdcard/nandroid folder)
You dont need PC anymore.
This recovery allow to disable the signature verification.
Ready to do this?
At your own risk!
Be sure you have MT6573 CPU inside your Star x18i!
Read Martin´s Blog, you need MT65x3 USB VCOM drivers and SP Flash Tool v2.1134.00 from there.
Download this ScatterFile and this Recovery
unzip Recovery.zip and rename "592zn.com_recovery_Star_X18i_MT6573.img" to recovery.img and flash it with SP Flash Tool.
To enter Recovery Mode press Vol up and Power simultanely