I've repaired a couple of games in my day, lemme tell ya, Crystal's the hardest to fix. My Crystal game has eaten through 3 batteries somehow. (probably because I dropped it once or twice ^^;)
Things you'll need:
A very small pair of needle-nose pliers
The proper battery, a 2025
A small Flathead screwdriver
Plenty of Tape, Time, Patience, and Superglue.
You'll notice that crystal has 4 little "dots" on each of the tabs, instead of the 2 that gold and silver have. This is the hard part. Get under the tab, and remove each of those dots. You'll know if it's removed when you hear a small "pop". Be careful NOT to remove either tab. But don't worry if you do. It can be fixed with superglue, but just... be careful.
When all 8 dots on both tabs are removed, carefully install your new battery. Put a dab of glue on the bottom (I use Gorilla brand impact-tough Superglue. Works as well as a soldering iron. DO NOT USE ELMER'S SCHOOL GLUE, THAT'LL RUIN YOUR GAME.) and put your battery in place, holding it there for at least 60 seconds. Don't use your hands, use a tool like your screwdriver or pliers, you don't want to be pulling skin off today. Next, put a dab of glue on the top of the battery, and hold the other tab in place, mimicking what you did on the other tab. Finally, use tape to hold everything together.
If your crystal works now, great! If it doesn't, please please PLEASE don't blame me for it. I simply gave the tutorial!