Responsive Tip-calculator App

What are you most proud of, and what would you do differently next time?

The app is working, but I didn't invest enough time to plan the structure of the javascript code.

What specific areas of your project would you like help with?

Since my JS code is very far from readable, my functions aren't decoupled, I need to do a whole refactoring.

What is a good "mental model" when you start to construct the JS code in a small -but not very small- project, like this? I thought It's small enough to just start writing, but later it created problems, and it's very time-cosuming to start again, after a certain point >> and this leads to a mess. So how do you make a plan before even start? (sorry for my english btw) Cheers, Mathias