Well, first u need to select the type of ball that it's more like the ball u want. See if the EA Sports model, or the Nike geo merlin, the Terrestra or the Fevernova is the one u want.
Then, go to disk.big or .fsh I don't remember, in the ...FIFA 2003\data folder, thenu select the image of the ball u want and export it as bmp.
Edit the ball u want adding the images of the parts of the ball in the image of the old ball, and when u think it's ready, save it, export it with Ball Raptor, and if u want to see a preview of the ball, try it in the game or use Oedit, just select the .o file of the ball in the data\balls folder, open it with o-edit, activate all boxes on the right, in the menu go to load texture, select the bmp, and then preview the texture with on of the otpions in Oedit, I don't remember what exactly it says, but it's something like texture preview or something like that.
Oh, I forgot it, u need to decompress the .o file, I think u can make it with EA GRAPH.