Before you can cut out the wall board in between the studs, you have to figure out where the studs themselves are.
An electronic stud finder is a great tool to use, but it's not always perfect. Here's another method you can use to double-check your stud locations:
Once you've located the actual edges of the studs, draw vertical lines on the wall to mark them. This way, you'll know where to cut when removing wall board later.
