I'll start with a premise and genre. Currently working on a dramedy/romance based around suicide helpline volunteers. So I started researching all about suicide helplines, what the training process is like, what types of calls they get. What the rules for volunteers are. etc etc.
Then I start to think of characters. I'll start thinking up their personalities and traits. Maybe write down a bit of a back story for them and a few lines of dialogue to help get a feel. I generally tend to use the voice and character of someone I know. So that when I'm writing for them I can imagine it's a real person talking. I imagine what they'd say and how they would act and it just flows more naturally when I can hear their voice in my head.
There's a guy I work with who will talk for hours if you don't stop him. And he loves history and the arts so he'll constantly be telling you random facts. One day he told me King Richard III had scoliosis and then said that Henry the 8th died of either gout or diabetes. I went away and typed up as much of the conversation as I could remember and have put a lot of it into my script. The character is just some nutjob who volunteers at the helpline and talks historical nonsense to the protagonist in between calls.
Then I just start outlining the plot points. Keeping in mind what my character wants and what he needs. I try and make sure each pivotal scene contains him making a choice and being active rather than just reacting to things all the time. And I try and make it so that the ending is the culmination of all of his combined choices.
While I'm planning I'll be thinking of scene ideas that would be cool, or conversations between characters and I just make notes from ideas as and when they come to me. Once the outline is finished I'll have a bunch of notes regarding scenes, characters, dialogue and it's usually enough for me to start writing with wind in my sails.