Open the angle finder. Most angle finders are about 8-inches long and they have two arms emerging from a rounded end. The finder opens by sliding the two arms apart.
Position one arm of the finder against the wall, and the second arm against the ceiling. Press so that each arm is flat and snug to the surface. Lock the angle with the angle lock. Turn on the digital display.
Read the digital display. The display will identify the true angle of the two surfaces. Most angle finders have a range of 225 degrees, and they often have a level built into the handle, so you can see whether the surface you are measuring is level or approximately level. Because the angle is locked, you can take your angle directly to the miter saw when you are cutting trim.