Selina Valen Brown
Just write. It's cheap - you just need a pen and paper or cheap/library computer. You can get free software these days. If you are working and have extra responsibility that's tougher - use a lunch break. But when I work full time I can't write at all except to record ideas.
If you have an idea let it out. Don't get caught up in spelling, grammar, punctuation! First drafts should be rough.
For the more methodical - create your outline and build that way.
Once you are ready for the real drafting stages just check what you've written and build upon it. Write down what you see in your head because that's all it really is. You are describing what you see in your imagination. There will be problems because you'll miss things, so, slow down the scene, replay it, or freeze it and record.
Once the story is there, you begin the drafting process and then editing process going over and over your story.