Pre-requisite: you need to know how to install and play a mod already.
You can start modding any special tools by simply modifying existing mods with Notepad. But I strongly recommend to use an editor with built-in XML support.
Best use Visual Studio Code as it comes with a plugin for Anno Modding.
Open it, go to Extensions (Ctrl+Shift+X) and search for Anno Modding Tools to install the plugin.
The RDAExplorer is a tool to extract files from the game. You need this to understand what’s in the game and reuse graphics.
If you have the space, I recommend to extract all .rdas into one folder beginning from 0 to the highest number.
Some .rdas have the same file again (e.g. assets.xml), the file from the highest numbered .rda is the only one the game will use.
If you want to do more than changing some values of existing mods, you need to research or refer to existing definitions called Assets of the game.
You can:
assets.xml with the RDAExplorer and use Visual Studio Code search (Ctrl+Shift+F)