Provide a clear example of a similar job you have done in the past.
Hi,
I have been writing stories for 6+ years now and have written eBooks and novels as well. This isn't a difficult job for me.
Proposal:
I feel a little awkward praising myself in a proposal, I believe in proving myself. So here's an offer: I'll write a 300 words story for you around any outlines you provide or if you want me to come up with them. If you like the story, we can continue with the project.
Cheers!
WordsAlchemy