This tutorial will let you create your own designs for the "Create-a-Sim" mode.
But be warned, doing this tutorial improperly could cause your game to stop working, so I would advise you back up your files before you attempt it.
I'd like to thank Snooty for giving me the basics of how to do this.

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First, open up the "My Computer" icon on your desktop and go to:
C:\Program Files\EA Games\The Sims 2\TSData\Res\GlobalLots\

Here you should see a file called CAS!.package
Copy this file, and then navigate to:
My Documents\EA Games\The Sims 2\LotCatalog\
And paste it there.

Rename it to cx_00000013.package or if this number is taken, rename it the next available number. Remember which number you saved it to.

Now open up The Sims 2, and go to the neighbourhood of your choice.
Take a look in the lots and houses bin, and you should find a lot with a strange name like jlghjidsfln or something similar. Place it somewhere in your neighbourhood, and go into it.
You will see that it is the set from CAS. Edit it in whatever way you wish, but keep the basic set in the same place. Then save, and move the house back to the lots and houses bin. Quit the game.

Now grab the cx_00000013.package file (or whatever you renamed it to) and move it back to C:\Program Files\EA Games\The Sims 2\TSData\Res\GlobalLots\
Rename the original CAS!.package to something like backup.CAS!.package
Now rename your edited lot to CAS!.package

Go back into the game, and open up the create-a-sim mode.
If you have done everything correctly, you should now be seeing your brand new edited CAS screen!

If you would like to see or download some examples of modified CAS screens, you can check out mine here.

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Comments or questions? you can contact me by clicking here.