Some writers outline and plan before they write. Others wing it. For those that plan, the question often arises: when is it enough? When can I actually start?
Some might say to simply dive in, start writing, and see how it goes. That may or may not work. Instead, I would develop a checklist revolving around what makes a good story and then test your outline, synopsis, plans, or whatever against the checklist.
A good checklist might include questions about character: do the characters have complex motivations born of passions, needs, and fears? Are the motivations clear? Do their goals conflict and create tension? Do they have backgrounds that adequately illuminate their passions, needs, and fears? Do they have complexity (i.e., layers of external and internal masks to reveal over time?)
When it comes to the plot, you might ask if there is a strong cause and effect relationship through the events and reactions of the story. Do the events of the story drive the characters along an arc and present them with an opportunity to change? Do the character's motivations push the story? Are the characters actively pursuing their goals and considering all of their options? (You don't want a character to skip what seems like an easy solution.) Does the beginning set up all the pieces on the chessboard? Does the ending surprise and satisfy?
This isn't a checklist to develop a story in the first place. It's a suggestion as to how a writer might evaluate the question of how ready they are to move forward and start writing, if they're in the planning stage. For those writers that wing it, the process isn't much different. The checklist just works as they go or during the revision process.
When it comes to planning vs. winging it (or seat of the pants, as it is often called), my only advice is for writers to be open to experiment and be willing to update their process.
My Work-in-progress, The Superhero Murders is at the tail end of the planning phase. I have all the pieces. I just need to fit them together. Unfortunately, a hectic work project is eating all my spare time and keeping me from doing much writing for the next few weeks, after which I plan to take a week off and get the story underway. I have my own checklist for that story and I'm feeling good about the answers.
Writer of speculative fiction. Current work in progress is an untitled dark fantasy novella.