I didn't want to dive in to development without setting a scope for the initial version. So I drew up some rough sketches and planned out the features that I wanted and the ones that I did not.
Things I don't want: spellcheck, grammar check, in browser experience.
I wasn't sure exactly what the best user flow would be so I built a single screen that I could play around with. Eventually, I narrowed things down to exactly what I wanted.