The first thing to do is solder a wire to the positive side of the Lily Pad LED board and bend it over the side so it will make contact with the side of the watch battery. You want to leave this a bit long so it will extend just beyond the bottom of the battery so you can adjust it later. Fill in the hole on the negative side of the LED board. You want a small dome to appear on the underside of the board- this will make sure it comes in contact with the top of the battery.
Now you need to make a holder for the LED and watch battery. You could carve it from plastic sheet or rod but since I wanted to make more than one I elected to carve a wax pattern and then cast duplicates in a rubber mold. The process for this is identical to how I cast my other resin Green Lantern rings so definitely read through that instructable. One tip I will make is that thin castings take longer to cure and can be more difficult to cast than thick castings. To get good quality castings I first heat the mold in a microwave for one minute on high and then I mix the resin for thirty seconds before pouring it into the mold. Once you have poured the resin let it sit in the mold for ten minutes before removing the casting from the mold. Using this method I get near perfect castings every time.
I stared by cutting a round disc approximately 14mm in diameter and checked the fit of that with the large hole in my ring. Then I carved an opening for the Lily Pad LED board, allowing a bit of room for the contact wire. The holder is designed so the watch battery will snap into place so it takes a bit of trial and error to get a good fit- I made a small ledge on opposite sides of the holder that help hold the battery in place. You really want to make the battery holder as thin as possible- there is very little room to work with in this ring so keeping the height down is important.
There are two notches in the side of the casting- the larger one is so you can slide a small flat bladed screwdriver in the holder to remove the battery and the other small notch is so you can make adjustments to the wire that runs along the side of the battery. If you bend the wire inward it will put a bit of tension on the battery to help hold it in place and make sure you're getting a good contact.
The LED board is held in place in the holder using a small piece of Kapton tape. This also covers the positive contact on the underside of the board so you don't short out the battery. You can use any kind of thin tape you like- thin mylar packing tape would also probably work just fine.
For those of you not wanting to go to all the trouble of making the LED insert they are now available from my friend on his Facebook page here- http://www.facebook.com/batjeepster.rings?sk=wall&filter=2#!/photo.php?fbid=10150256885538462&set=a.495535658461.270748.289792643461&type=1&theater