First of all, download this file: runtime_feeds.json (it's also attached to the bottom of this page).
Next, open your GameMaker LTS data folder:
- Windows users: %programdata%\GameMakerStudio2-LTS\ folder in Explorer
- macOS users: /Users/Shared/GameMakerStudio2-LTS/ folder in Finder
Then move the runtime_feeds.json file you just downloaded into this folder.
You will have a file of the same name there already, so say "Yes" to replacing it.
Now install and start the version of GameMaker we linked to on the GMC thread which sent you to this page. (No special instructions for that bit - however and wherever you normally install to is fine.)
That GameMaker version will be able to download its matching runtimes on startup and once it has loaded up you will also see your Preferences now has two feeds for your runtimes:
And that's you all setup and good to go.
If GameMaker did complain on startup that it couldn't find your runtimes and you only see a "Master" feed in your Preferences, then the file was not copied into place correctly. Please try again.
If you want to always take part in these LTS "early looks" in future, you only need to do this once - just leave the file in place and it will alert you to any new LTS runtime releases as they are made available.