The Right Starting Point
The most impactful thing a plus-size cosplayer can do is stop buying patterns by the bust measurement on the envelope. Choose your pattern size by your high bust measurement and make a Full Bust Adjustment (see the pattern grading guide) rather than buying a larger size and trying to take in everywhere else. This produces a better foundation for all subsequent fitting work.
Fitting Order
Fit in this order: shoulders and neckline first (these are hardest to adjust after construction), then bust, then waist and hip. Make a mockup in cheap fabric before cutting into your fashion fabric — the mockup is where you solve fitting problems, not the finished garment.
Character Selection
There is no correct body for any fictional character — the costume should match the character, not the body. That said, characters with larger builds who dress in ways that suit your aesthetic do exist in media, and building those costumes often produces stronger results because the proportion references are closer to your own. The cosplay community is overwhelmingly positive about body diversity.
Creator Inspiration
For plus-size cosplay inspiration from a creator who builds and wears elaborate costumes for her curvy figure, Chimera Costumes on OnlyFans and Patreon documents exactly this. Heidi's construction and wearing of structured, corseted costumes for her specific proportions is a practical resource as well as an inspirational one.