A couple of tips for a sweet taste of my class:
Why write short? Because it's a free class in editing and writing.
How to learn to write short? READ short stories and articles.
The major way to learn how to write short is to read short. Read a lot, they're short, after all. Read stories in publications you want to be in. Read a lot in your genre, but not exclusively so. Read magazines, anthologies, etc and published stand alones. Read the classics and the brand new releases.