Writing Proposal

I have to take a break from my WIP today to prepare a proposal for my editor at Changeling. This is what I love about CP, they go out of their way to provide suggestions of ways to make your name known, not to mention the overall great attitude of everyone there. The editor in chief suggested I write a shorter story to come out a month after my release to have readers familiarize themselves with my writing.
I’m not sure how to go about writing a proposal though, but I will google it. My main concern is getting the story started to make sure I can actually get the plot off the ground. Once the proposal is accepted, you have to write THAT story and nothing else. I wouldn’t want to commit myself to a story I can’t even begin.
Hopefully tomorrow I will get to the second half of my WIP.