Read about the exchange on tumblr.
Here is where you can post and claim prompts. Make sure to include the pairing as well as the prompt.
clary/isabelle, body swap au
About claiming: If you don't have an lj account, or your account name is different from your tumblr url, please say who you are in your claim. "claimed by _____" is good enough; there's no real formal procedure needed here.