However, before you start developing, I recommend creating a functional and technical specification—it’s really the best way to outline what you want before coding.
Example of a spec: Multi-utilisateurs - #6 par pierre-gilles
Personally, I use https://whimsical.com for mockups; I find it practical. The idea isn’t to create a pixel-perfect design but rather to see together what will be added to the UI so that we have an idea of what we’re doing before diving into development.
The advantage of Whimsical is that it’s simple and doesn’t push you to create a bloated UI.
For the technical specification, the goal is to explain technically what we’re going to do before doing it (database modeling, API routes, etc.).