I'd recommending downloading PSX2PSP app first which can turn all of your .bins and ISOs into compressed PBP eboot files, this will save you about 35% of your space.
For multi disc games like FF or MGS, you need to convert each disc to a PBP file 1 at a time but save them on the same folder.
Here is the trick, if you just save the converted eboot file to the same folder, you'll overwrite your previous file because they all do the default name of EBOOT. So once you convert 1 file, you need to rename it, like EBOOT1 or 2 or 3 or so on.
So for Final Fantasy VIII, I'd convert the ISO for disc 4 first. Once its done, you'll have a file folder created by the game's SLUS ID. This ID number will be listed in the PSX2PSP app. Once disc 4 is converted, go into that SLUS folder, rename the EBOOT file to EBOOT4. Convert the ISO for CD 3, then rename it EBOOT3. Keep the name convention until you are done.
When you are ready to add the game to the PS1 classic, browse to that SLUS folder, click and drag to highlight all 4 eboot files then click to add game. When you play the game itself on the PS1 classic, you'll get the note that its a multi-disc game and to switch discs when notified.
The best thing about the eboot files is you don't have to worry about cue files, or configuring the ini file, or having to have any of the additional bonus cds added like those Working Design games so the INI config can recognize the right disc.
