Alright, no one had a preference on FF:CC Clavat or B&S Gunmahye, so I'll do step by step explanations on this one instead. It has the same techniques, except on a grandeur scale.
I want to say upfront now, that fabric is not my friend. I love making armour, I love making props, but for the life of me, fabric is my bane of existance, and this year is dedicated to working on very complex fabric designs and detailing. I like the challenge, and it never hurts to improve on a sore spot! So please understand that there are probably several easier ways to go about this! Find what works best for you. :)
Here we have a rough pattern in the making, it's Gunmahye's corset, I sketch out a template similar to when I make armour, and will fine tune it until it lays flat. I wanted to show you this step, because it's usually neglected and you see the pretty finished version instead. I do pattern by eye/hand coordination, so trimming and modifying happen throughout the entire process. Since this isn't a cardboard prop, I'll need to add a seam allowance on the fabric pattern, so I can actually stitch it together. You can add it to the pattern right away, or just add it as you cut the fabric. The first option gives you a better result and straighter lines, unless you feel confident in cutting.
If you wanted this for a actual cardboard armour piece, once you figured out the pattern, it's straight cutting, and you'll just need a backing to strengthen it inside. Patterns are important, they lay the grounds for your project, and it's a hundred times easier fixing a aesthetic problem, then having to make the entire piece from scratch because something didn't line up or fit properly. Construction is the easy part~ :D