Hi! There are no rules for measuring, as long as you try to make it as consistant as possible. You can do it however you'd like, but it's a good idea to write down notes to yourself for where you measured, like "at belly button level". I happen to have some prominant birth marks that are my land marks.

For trunk measurements, it's a good idea to check a mirror to make sure the measuring tape is parallel to the floor. I usually exhale comfortably for "lung capacity consistancy."

Have fun!