Identify the case back tool you need. Snap-off backs will be completely smooth. If your watch has a snap-off back, simply slide a case back knife under the side of the backplate and apply upward pressure to snap it off.

Screw-off backplates will have small indentations, as seen on my watch. I am using a wrench to twist the backplate off. Use the round slider to adjust the width of the tips, then tighten them into the indentations on the watch back and twist until it loosens. This exposes the mechanical movement.