Steps in the video 

Step 1: Put on your favorite unlined bra
Step 2: Wrap measuring tape around your ribcage just beneath the lowest edge of your bra. This is your band size. 
Step 3: Move the tape measure to the inside of the underwire at center front. Measure over the fullest part of your breast to the wire at your underarm. This is your bust size. 

Next, with the two measurements from Step 2 and Step 3 in the video,
refer to the chart below to determine your bra size

Next, with the two measurements from Step 1 and Step 2 in the video,
refer to the chart below to determine your bra size