You as the designer are not going to come in the box. Clear, precise, quick to comprehend rules are essential.
Learn what to include, how to prototype, publish and play test your rules.
Written
Visuals
Diagrams
Photos
Testing your rule book is essential - can your players understand how to play without you. In the beginning you will need to help them a lot. Then take notes about what you needed to explain and keep editing your rules until you can remove yourself from the game play and the players can comprehend the game on their own.
Theme
Game Overview
Components
Objective
Setup
Game Play
Game End Conditions
Dancing Giant Games Intro to Rules Writing
Best Practices for Creating Rule: The RuleBook Cookbook
You can publish a PDF for a print to play which can also be helpful for players who want to learn about your game before they buy and you can also print a rule book that goes with the game.
Have in game reminders of the rules. The game Root does a great job of this with a large cardboard piece that reminds everyone of their rules, especially if it is an asymmetric game where players have different goals.
Have a quick play through example.
Look for the best size paper book for your game
It must fit into your box within your pricing constraints
Using Photoshop, Illustrator or InDesign, (or similar like Photopea but Krita is not a good choice), craft your multipage document
You could also publish through InDesign as an online document.
Use linked folders of art and text into indesign for quick change needs.
I recommend to put version numbers on your instructions on the first page, as you will have many and do not want to get confused.
Review this example for PowderKeg
Instruction Booklet - Version R - with Game Play testing questions for note taking. Scroll to view.
Medium Booklet: https://www.thegamecrafter.com/make/products/MediumBooklet