Digital Sliding T-Bevel/Angle Finder

I suggest you actually go to the store to look at the tool. This is a great tool. It is clearly trademarked "HUSKY"

used it for installing stairs

I also ran into this problem. The tray where the battery is supposed to go was empty and no sign of a battery anywhere else in the package.

Hold the tool horizontal with the metal blade to the left and looking at the screen.To the right of the screen there is a rectangle recess. On the left side of this recess find a curved tab. That is the "handle" to the battery holder. Insert a thumbnail or straight blade screwdriver (or similar tool) into the indentation of the tab and pull to the right. This will pull the battery out. Remove the spent battery and insert the new one with the plus/positive side down. Slide the holder with correctly inserted battery to the left until it snaps into position. Mine automatically turns on when I do that, just saying. Oh, your all done!

Close blade. Push zero button.

Yes. Use it to measure the angle and transfer that to your miter saw. That should get you an accurate fit.

Yes, mine was inside with a piece of plastic over the battery.

If I'm reading this correctly you mean a curved part jointed to a straight part? You can theoretically get the "angle" with this, but you would need a curve measuring tool, as you're technically not measuring an angle joint, but a curved joint. Usually the best method for measuring these is grab a piece of cardboard or something, hold it up to the joint, and trace it, so that you can then copy that trace.

Save your receipt for replacement.

Where the screen is, at the bottom is a sort of square gap. In this gap (right below the screen) is a tiny round edge. You slide that down, battery goes in, slide it back up inside. So to clarify, the battery basically sits right under the screen.