One of the questions most frequently asked by new cosplayers is, "How do I start?" Whenever I get messages or comments with this question, I have to sit there and stare at the words for a while. Sometimes I'm not sure I understand what they're asking. I've been cosplaying for so long that all the prep work is almost second-nature, and my "start" of a project is buying the fabric and cutting into it, but that's not really the beginning.
First I choose what character I want to be, and then I study them. I stare at reference pictures, look up multiple angles and detail photos. I make sketches, even if they're not always good, to familiarize myself with details. I break down the costume into pieces and layers, and draw out each one. Then I look at material options, fabric types, what fabric folds and flows the way the character's does, what fabric the character would have access to, and what needs to be bought and where.
After you've got all of that down, and you know what you're getting yourself into, the fun starts. If I'm using fabric, I need a pattern that's at least close to what I'm making, and I'll probably need to make a mock-up to alter. If I'm making props, I study different techniques and materials to find the easiest and most cost-effective way to make the cleanest-looking prop possible. Probably do some test pieces first.
Then I just.. Do it! After collecting materials and resources, it's time to get things done! Pick a part, and start working on it!