You can also take the key assembly apart, removing the key shaft from its base. Use a small punch or a fine screw driver and tap the pin out through the other side with a hammer. Only light hammering is required. But it is easier to leave the key assembly together, and transfer the whole thing to your new case.

Now you have completely disassemled one key fob - your old scratched up or damaged fob. Take ALL of the parts and put them into a zip-lock baggie. This way you won't lose anything (like that tiny transponder chip) while you work on the other fob. Also, this will keep you from confusing which transponder is which.

It is also a good idea to mark the OLD (and working) transponder chip with a marker, so you can tell the difference between it, and the transponder from the fob you bought on eBay.

Now that you have some practice and experience, get your spiffy undamaged eBay fob, and disassemble it.

Take the key assembly, circuit board, and transponder chip from your OLD fob and put them in the NEW case.

To assemble the case, simply reverse the above proceedure.

Here's some assembly tips:
1) put a little dab of Elmer's glue on the transponder chip before you put it in the new fob. This will hold it in place while you reassemble the fob, and keep it from rattling around inside the case, if you don't have a white retainer.
2) When putting the circuit board back into the case, be sure that the metal flap (battery terminal) slides neatly into the slot in the case designed for it. If you miss, you could bend the terminal or even break it off the board. Then you'd be screwed!