Bought a pregnancy planner 🤗

I bought a pregnancy planner to keep on top of all my feelings and symptoms of my 1st pregnancy. I’m only 5 weeks and for week 5 the book asks for bump measurement...well I’m obviously not showing yet so I was just going to measure myself anyway and put that down but now I’ve read to measure your bump you measure from top to bottom so under your boobs to under the bump...so I’m completely confused as to how I should measure my tummy now with now bump? Anyone kept a pregnancy planner and come across this too or anyone just got any ideas ☺️ thanks