Designing Kitchens and Bathrooms - How do I cut out access in a wall?

I have to get behind the bathtub faucet for removal. Unfortunately there is no access panel so I'm going to create one I guess.
But how do I cut the wall without tearing everything up? What tool do I use? This is in a mobile home, I think I can probably punch through the wall pretty easily but I would like to make some clean cuts so I can hopefully have something decent to work with when I cover it back up.

Thanks in advance

Locate the position fairly well. Use a framing square to draw a square around the area. Use a razor knife with a sharp blade to cut out the sheetrock/wallpaper combination. Once removed, you can perform your repairs. You can make a picture frame to cover over half the cut portion, hot gluing the piece to the frame. You can either screw the frame to the existing wall or do what I do, hot glue the frame to the wall as well. If you have to get back in there, it can be removed, but if not it will remain tight.

Thanks so much for the tip, I'll give that a go in the morning.

Cheers!