Wrap a measuring tape around your back, under your arms. Measure just underneath your bust, where the wiring of your bra would normally sit. Make sure the measuring tape is parallel to the ground. Input your measurement (in cm) in the field “Band cm”.
Place the measuring tape around the fullest part of your bust. You won’t need to pull the tape particularly tight, it just needs to sit flat, but you’ll still want to make sure it’s parallel to the ground. Input your measurement (in cm) in the field “Cup cm”.