Getting down to writing can be hard; you need some way to get into the mindset. As such setting up some sort of ritual to make the shift can only help.

You need to have passion when you write, be it love or hate; if you don't it will show in what you do. Also, if you lose the passion don't be afraid to move on. Just make you some sort of connection to what you're writing.

You need an outline like you need a road map: It tells where you're going, what you need to do to get there, and makes sure you don't get lost. Make sure that you have one.

You are not an imposter; don't let anyone think so including yourself.

Basically, just write. Don't just plan, prepare, or fantasize about being a writer, but write.