What you do is you build it on your phone.../data is where downloaded apps are, a lot devs don't even include /data but some do. Good example is blackdroids ultimatedroid. The reason it has all those Market apps included in it, is because he uninstalls all his personal apps except the ones he wants you to have in his ROM.
then make a backup (but wipe cache before you do it!!) And only backup boot, system, and data.
once you have the backup on your card, copy it to your computer somewhere, and make sure you something likeWinRAR. With WinRAR you right click your backup file, and select the option that something like "send <filename> to <filename>.rar archive", or you can simply zip it too.
The point is, when the phone backs up via nandroid, it creates the .tar files then