Step 1: What Do You Need and How Do You Do It?
This guys are great for this project, they are thin, cheap and with a hard surface resistant to scratches, You can reuse and relocate them and does not leave any sticky glue behind.
A exact-o knife or scissors
A template of a pickguard in piece of paper or cardboard (check online there's lots of them, reuse an old one or make your own) make sure that fits on your guitar.
For starters don't peal the mouse pad yet, first of draw the template over the pad, cut the shape out and then measure your new pickguard over the sound board of the guitar, to see how it looks, later just peal and place it and your good to go.
The nicest thing is that if you don't like the location or the shape you can always modified it of reuse it in a different guitar, and it will protect your instruments from scratches.