Go into the room where you are installing the paneling, and walk to the right side, which is where you should always start the measuring process.
Place the tape measure on the right side of the wall approximately one to two feet above the floor. Measure across to the other side of the wall, and note the measurement. Repeat the process midway up the wall and again at the top, and note each measurement.
Walk to the farthest left side of the same wall, and measure from the ceiling to the floor. Repeat the process in the middle of the wall and on the right side of the wall, and write down each measurement. Each measurement might be different depending on the wall shape.
Examine the measurements for the irregularly shaped wall, and find the one with the longest width and length, which is the size of the paneling that you need.
Move to the next wall to your left, and measure it in the same manner, writing down the measurements. Continue working your way around the room until you have measured each of the irregular walls and figured what size paneling you need for each wall in the room.