Steve, I sent this in an email yesterday, but your Ford dealer bounced it back as spam... so here goes.
Download a PSP template here in the downloads for your favorite mod. Although, most are created using PSP9 so don't know if they will open for you or not. If not, open the PSD version.
Now when you get it open, you want to change the colors of the car down around the 'base layer'. Use the help section within PSP to get a grip on using 'layers'. It's just as it sounds... you work with 'layers' so you don't mess up whatever your working on around you.
Example: I want to add a number to my car... I find a 'font' that I like and I make it the right size to fit the car. Now I want to be able to move it around to adjust it, without moving the decals around it. That's why I use 'layers', so each one can be adjusted/moved/drawn separately while I'm 'painting' my car.
Now add some logos and sponsors... use the move tool to move them... you can resize them in PSP with the 'warp' tool I believe.
When you get something you like, save as a tga file to import into the game. You will want to save it with the 'alpha layer' turned OFF to make your file a decent size.
That's about as basic as I can get it.
Give it a shot and try a few times. It takes patience to learn how to use it, but if you use the 'help' section in the software it will do just that... help you.