Taking Accurate Measurements
Taking accurate measurements is the most important task when working of a premade pattern. Working with measurements that aren't exact means you'll spend a lot of time and effort making a costume that won't fit properly.
PATTERN SIZES DON'T COINCIDE WITH CLOTHING SIZES! Your pattern size will likely be 2-6 sizes larger than your clothing size. You will need to take measurements to find the appropriate pattern size. Pattern sizes also vary a bit depending on the manufacturer, so write your measurements down for the future!
It's best to have someone take these measurements for you to ensure accuracy, but it is entirely possible to take these measurements yourself. Stand in front of a full-length mirror to make sure the measuring tape is laying flat for each measurement.
Bust. This is not your band size! While wearing the bra you plan to wear with the costume, measure your bust at the fullest part of the cup. Make sure the measuring tape is completely parallel to the floor.
Waist. This probably the easiest measurement to take. Your waist measurement should be taken at the smallest part of your waist, with the measuring tape perfectly parallel to the floor. You should be standing straight while taking the measurement, as your waist morphs when you're sitting or kneeling.
Hip. Fashion is tricky. Your hip measurement isn't taken at your actual hip, but 6-8" lower at the fullest part of your butt. Once you have the measuring tape lined up in the back, be sure the front lines up so the tape is parallel to the floor.
Inseam. Pants patterns will usually call for an inseam measurement. This measurement goes from your crotch to your ankle. If you don't have someone to take this measurement for you, you can step on one end of the measuring tape to hold it in place and bring it up to your crotch.
Other. There are a few other measurements that are sometimes needed, depending on the garments you're looking to make. Below is a diagram featuring a few more measurement guides.
Neck: Measure the circumference of your neck at your Adam's apple.
Sleeve: Measure shoulder to wrist in a straight line.
Thigh: Measure the circumference of your thigh about halfway between your crotch and your knee.