Required Programs:
'UMDGEN v4.0' (Do a google search for it)

How to convert an ISO file into a CSO file:
1. Download and install UMDGEN v4.00
2. Open UMDGEN and open your ISO file.
3. Go to the 'UMD Properties' tab and click the 'Optimize' button.
This will optimize the order of the files and remove un-needed files
Some games may not work properly when optimized and may freeze on a black screen.
If this happens to your game, do not optimize it and skip this step.
I recommend to keep the original ISO file until you know the game works properly on your PSP.

4. From the Toolbar menu, choose Save -> Compressed (*.CSO)
5. Choose a folder and a filename to save the compressed ISO to and click 'Save'
6. UMDGEN will prepare the files and then compress it to the CSO format.
7. Once its finished compressing, you will have your CSO file.

Convert a CSO to the ISO format:
1. Open your CSO file with UMDGEN.
2. On the toolbar, choose 'Save as' -> 'Uncompressed (*.ISO)'
3. UMDGEN will prepare the files and save it as an uncompressed ISO file.
4. Once its finished saving you will have your ISO file.