Do you document your app idea before jumping into code...

Yes, all the time, it's like planning a project, and it helps you not miss out on details, or anticipate some complexities while coding. Agencies like https://coaxsoft.com/solutions-m... are now using similar methods to get the job done better

we note everything down, don't get into it until we have a full understanding of how each feature will work. saves a lot of time from getting into it without knowing exactly and running into an issue or it not possible the way its wanting to be intended, etc.

It personally took me over 2 years to document everything. I started with Balsamiq Wireframe as I am not a dev myself. My goal was to make the design of the interface, and more importantly, the technical aspect of the app. My SaaS is built for the agriculture industry. If you can't provide an extended amount of data to simplify the process, the dev will not get it. Now I am working with two devs, and as @krishna kumar said, get all your documentation in order, every detail is crucial when you want to integrate some complex features. Hope it helps!