1.) Unpack the emulator to some folder.
2.) Copy the PSX bios to that folder (or you can make a new folder called bios )
3.) Start Xebra.
4.) Select "File->Open->OS ROM Image" and select your PSX bios.
5.) Exit. (now you have 3 new files: BU00, BU10 and OSROM).
6.) Restart Xebra.
7.) Now select "File->Open->CD-ROM via SPTI" or "File->Open->CD-ROM Image" (or in other words ISO image).
7a.) If you select "File->Open->CD-ROM via SPTI", select your CD-ROM and select any file on it.
8.) Go to Run and select either Run1, Run2 or Run3 and wait a while, the emulator should start now.
Here the original help:
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