My watch has a screw- in back on the watch case. It is possible to order a universal spanner wrench for around $10 US. I wanted to make my own spanner wrench so I can replace the battery without needing to get to a store that can change it for me.
In the photo you can see the indents for the spanner wrench so it can unscrew the case back.
Step 1: Use a Mending Plate
I used a 1/2 x 2 inch mending plate. These are available in any hardware store. Any thin piece of steel would work, too. (The photo is from Google Images.)
Step 2: Mark the Area to Grind
I like to make markings for cutting or grinding steel on masking tape. The markings are easier to see. Cover the mending plate with masking tape. Mark the edges of the spanner wrench indents.
Step 3: Rough Grinding
Grind away most of the steel that will need to be removed, but do not go too close to the marks for the indent edges.
Step 4: File to Fit
Use a good file to remove the amount of steel required for a very precise fit on the back of the watch. Check your work frequently.
Step 5: The Wrench on the Watch Case
If all was done properly, your new spanner wrench fits the indents very neatly. Remove the tape. Grasp the spanner wrench between your thumb and first finger. If the case back is on very tightly, you may need to grasp the mending plate spanner wrench with a slip-joint plier. Hold the watch firmly with your other hand and twist in a counter-clockwise direction.
Step 6: The Back Is Off.
Here you see the back of the watch case removed. Replace the battery. Be sure to get all gaskets, etc. back in place. You may want to turn the back of the case counter-clockwise until you feel the threads fall in so you do not jamb the threads by cross threading.
Step 7: Easy to Carry
The last time I changed my watch battery I slipped my spanner wrench in to my wallet and changed the battery immediately after purchasing it at a store that does not change batteries for customers.
You can also see that I cut a second profile on the other edge for a watch with a different size back. Now my one wrench allows me to change the battery in a second watch I came to own.