Compared to other garments our corset measurements are actually pretty simple. We only need 3:
Wearing a comfortable bra (not a pushup or padded) measure around your bust at the fullest point.
Measure around the smallest part of your torso. It's probably nowhere near where you wear your pants (as the above picture illustrates) so ignore the conventional thinking of "waist". If it helps tie a piece of elastic around your waist and do some side bends and toe touches so that it settles into the smallest area, then measure around the elastic.
For this measurement we're going to do things "wrong". Instead of a conventional sewing hip measurement, which is taken around the largest point of your thighs/butt, we're going to take a measurement of the hipbones. Few corsets reach as low as your natural hip measurement, so you want the measurement to be close to where the bottom of the corset will lie.
Picking your pattern size:
Choose your pattern size according to your waist size. If your bust or hips are a size or two smaller or bigger you can "blend" the pattern: that means marking the size of your waist using your waist size, and marking the separate size of your bust using your bust measurement, and in between connect the two with a smooth line. You can do the same with your hips if they are radically different.
Measuring the pattern:
Do yourself a favor and measure your chosen pattern along the waist. Take each piece and measure the waist on it, subtract the seam allowances, and add all the piece measurements together. It should total a number 3-6" smaller than your natural waist size. If it's not then you have the wrong size, and keep trying until you get a measurement that is closer to 4". This is especially important on patterns that are known to run big or small, like Simplicity 7215.