I've answered this a few times before, but here it is again.
Warning: This isn't for the uninitiated. Mod porting is complex stuff.
Copypasta ahoy:
1. You need CSZ Tools from the modding tools download page of tribute site. This allows you to extract the data files used in CS PSP. Proceed to do this and then copy your PC data directory contents over the top of it.
2. You cannot use 24-bit bmps or pbms in psp ports, it must be 8-bit or lower only. If you rename all pbms to bmps using CKRename (look it up, awesome mass-renaming tool) then in windows vista and up you can add bit depth to the detail view when looking at files and can easily open those files in gimp and convert them to 8-bit (Image > Mode > Indexed) bmps. Convert back to pbms.
If you have Windows XP or lower just look for images with large filesizes.
However if you made the mod yourself then you should know what pbms you edited. Or hey, skip this step if no images were changed.
3. You need to make sure that the starting events, maps, positions, stats, etc are as default (check this with cave editor). If they are unmodified then ignore the rest of this step. If they aren't then make them so and modify the default starting event to take you to to mod's original starting event, map, and position with a <TRA. If the default starting event is already used for something in the mod then something like this should suffice:
#0200
<FLJ4620:0201(Original event 200 tsc goes here)
#0201
<TRAWWWW:XXXX:YYYY:ZZZZ
And in the map and event you asked for use <FL+4620 at the very beginning before anything else. Feel free to substitute this with your own choice of flag number.
4. You cannot have map numbers higher than 94. You can use the recent version of Booster's Lab to delete unused maps and move maps if you happen to have too many maps. It will automatically rewrite the <TRAs for you. If you aren't sure what's used by the mod then you should generate a <TRA listing using same said editor. Make sure that your splash intro map is map #72 and your starting map is #13. Save all this of course.
5. Open the exe with XVI32 or the hex editor of your choice and look for the mapdata for the mod in the exe. Look at maps.dat for an example of what it looks like. Some editors store maps in two places and you'll need to find the mapdata relating to your mod. Extract your mapdata (use Edit > Delete to/from cursor on the byte before/after your mapdata), truncated to the same length as maps.dat and save over maps.dat. Noxid will be adding an option to extract this data in the next Booster's Lab so if you find this step confusing then you could just wait till then.
6. The lounge bug: look search your mapdata with a hex editor for the string "lounge" in that exact case. Rename it to "Lounge". This problem doesn't crop up on windows, but on more case sensitive systems such as the PSP the map won't load and the game will crash.
7. Copy maps.dat to the same folder as an unaltered CS psp eboot and run this tool (which should also be in the same folder) to inject it into the eboot:
http://www.cavestory.org/downloads/inspsp.zip
If you wish to see the output of the tool (because windows is too fast to terminate dos applications) then create and run a .bat file with the following contents:
inspsp.exe
pause
Remove these extra files afterwards.
8. Recompress and repack your data.csz file with CSZ Tools.
9. Test the mod.
10. Use PSP Brew to add your own icons and background images to your eboot.
11. Test the mod.
I'm assuming that your mod has no custom orgs or weapons.
Also I could simplify these instructions a bit more if I knew how extensive your mod is.
