How to install VSCode and OpenXCOM extensions

TOC

Install VScode itself
Install the extensions
Optional: Change validation rules from OXC to OXCE
Open your mod in vscode
Open a rulefile and get modding

Install VScode itself

Go to https://code.visualstudio.com/download and download VSCode for your platform
Install and run it
It will now look something like this:

Install the extensions

Open the marketplace (hit CTRL+SHIFT+X or click the button on the left with four squares):
Search the marketplace for ext:rul and install both OpenXCOM Ruleset Tools and OpenXCOM Ruleset Linker by clicking the green install button
Wait for installation to finish

Optional: Change validation rules from OXC to OXCE

This is needed if you're going to be modding OXCE.

Open VSCode's settings by pressing CTRL+, or File => Preferences => Settings
Type ruleset validator
Click oxc and change it to oxce

Open your mod in vscode

Now go to Open Folder
Navigate to your mod and select it

Open a rulefile and get modding

Navigate to one of your rule files in the explorer on the left, open it and get modding!