Imperfect.
Not polished enough.
Rough.
And so on.

None of the above is a worthy excuse to delay starting.

Just start.

You don't optimise an idea that's still in your head. You optimize prototypes, work in progress etc.

Never be ashamed of your chapter one. Never be ashamed of your first working draft. Never be ashamed of your initial output, there's absolutely nothing wrong with it. Just continue working on it.

But start.

Start scrappy. Start unpolished. Start imperfectly. 

Just start.