How do you use PayPerPost.com?

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I joined PayPerPost.com and submitted my blog a few days ago. Now how do I write for them? I've gone to the list of oppertunities available, but how do I choose one? Or rather how do I let them know which one I want to choose? I don't see a box or link or anything. Do I just write the blog post first and submit it then choose which oppertunity it is for?