This is the tricky part. I couldn't solder the batteries onto anything. They could only touch the metal. This is because to recharge, I had to put the batteries back into the cell phone, where the plug in is. That's the reason for the tissue paper and wood.
Put the wood in the cassette case to make the batteries fit in tight so they don't move around. I also used a ton of solder so if it did move around, it would still make contact.
Once the batteries were in place, I put the tissue paper in to keep the batteries from moving around.