1. First of all you're going to need to download and install the theme builder, this can be found here.
2. After installing and opening the builder, click the New Theme button in the lower left corner and then build your new theme from scratch.
3. Now you are going to want to click on the Backgrounds tab. HD Backgrounds must be 1920x1080 in size and SD are 640x480. To get these resolutions you will either have to be lucky enough to find the image you want in that size or use an image editing program such as Photoshop, Gimp etc. to make it the correct size (I'm sure theres a few browser based programs that will do it for free too.)
4. Once you have your desired image in the correct size, select either browse for HD or SD background depending on you TV/Monitor that you use your PS3 with.
Note: Sound files must be either .vag or .wav format.
Adding Customized Icons
5. This next step you will be adding custom icons. Switch over to the Icons, Colours and Fonts tab. Here we will mostly be focusing on custom icons as fonts and colours are pretty basic. If you choose not to have custom icons, it will just default to the original XMB ones.
Icons must be of 128x128 size. The trickiest part of these is that if you want your icons of a unique shape - not a 128x128 square, you are going to need to manipulate the image a bit. In most cases you are going to need to make the background of the icon transparent. Photoshop is probably the easiest way to do it, but Gimp is free. The following is a tutorial for making transparent images in Gimp.
6. Uploading icons is pretty much the same as backgrounds except you've got about 10 or so icons to add instead.
Now that you have your icons and background sorted, it's time to put it all together.
7. You need to go back to the Theme Information tab and fill in the author box etc. (you don't need to fill in the URL and Version ones) and then click Generate Preview. From there you can see how it is going to look.
8. You now need to save your theme. This will build your theme so that you can install it. You need to locate the theme on your computer. Mine is located in My Documents under My Playstation Themes.
9. Put the .p3t file onto a flash drive or portable harddrive in a folder setup like this F: PS3THEME
Note: It must be in capitals exactly as shown (except the F: may vary)
10. Finally, you just plug your portable storage into your PS3. Enter Theme Settings under the theme tab. Click on theme and then install. Select the theme you just made and select it as your theme.