Anyway, once you have a Polyvore account, you just need to think about an outfit you want to put together and search for what you want to include. For example, you can search for "green dress" for a Link costume or "black feather earrings" for a Castiel costume (like this one). To add an item to your virtual closet, you have to like it. Polyvore has a huge number of items in their database, but if you don't see what you need, you can search for it on Etsy or Google and add it to the Polyvore database with a tool in your bookmarks bar.
Then you can choose "Create Set" and pull from the items you liked. You can also search for items as you create your set. It is incredibly easy to resize items and move them around. Once you get the process down, making lookbooks actually becomes a little addicting. So much so that I can't stop. I'll share them here once or twice a week, and you can keep up with them at my Polyvore page.
Here's one of the first few I put together, Link from Legend of Zelda!
I decided to pretty much ignore prices when I add things to my lookbooks because they're meant as inspiration. It's not too difficult to take the ideas in the Link cosplay and find affordable versions of everything at stores like Target and Forever 21.
If you have any requests for everyday cosplay lookbooks, please let me know in the comments!