When taking your measurements, don’t wear anything baggy or bulky! Leggings and a vest top is a good choice.
A good quality plastic tape measure will be most accurate. The fabric ones sometimes stretch a bit, especially over time.
When measuring from your waist or your hips, dangle the tape measure so that the start of the tape measure is at the bottom.
Don’t pull too tight! Keep the tape loose. You want to be able to breathe! It’s always easier to make something that is slightly too big smaller than the other way round.