The first and most important thing to do is to jot down the idea on paper.
When we get the idea in our mind, we think that we are clear with the concept, but that's not true. When we start writing the idea on paper, many new points come up. The plan in our mind is never structured and organized. Writing it on a piece of paper will help one to hold the idea, which will ultimately help to clear out self-doubts. Another thing is that this activity will do to let you know your situation. No idea is worth if you can't explain it. And when you write the idea, you tell it to yourself. If you successfully explain it to yourself, you will be able to explain it to others.
Once you complete writing down the idea, you will be able to analyze your idea more clearly. The paper will start asking questions from you. Those questions will help you expand and elaborate on your idea.
Tip: Do not try to put any creativity in your first write-up. It will make things more complicated. Please write down your idea in the most basic form as it is in your mind.