Right... I'm not quite sure how long it will take to make a PSP theme, but here it goes...
Go to
http://psp-themes.net/ then click on "PSP Theme Creator" if you are using Firefox, it should come up with something like this:
If you are using Internet Explorer, just click save file.
Once you have done everything, and if you have WinZip, then open up the PSP theme creator. Press I Agree on the WinZip (if it comes up with the box). Then double-click on the "themeconv.exe". No need to read "psp-themes.net.txt". It should come up with "Custom Theme Converter". The title box is the name of your theme. Product ID is your PSP ID. To see what it is, take off the battery compartment, and take out the battery and in a small white box, it should say "Serial No." put that in the Product ID. The final box, is the Version of your PSP. To see the version of your PSP, go to Settings -> System Settings (turn on your PSP...) Then go onto System Information. It should come up "System Software", "Nickname", and "MAC Address" My System Software is Version 5.02. Next is the Wallpaper (this will be the background of your PSP) you will need a .BMP file by 480x272 and 24bit colour direct image. Next are the icons, change them if you want to. First level icons are the icons within the main icons. And then you're basically finished! Don't worry about anything else!
Have fun with your PSP theme!