Once per computer. About five minutes.
Your browser will try to stop you. That is expected — it does this for every file of this kind, not because anything is wrong.
In Edge: the download shows a warning. Click the ··· next to it and choose Keep. If it asks again, choose Show more, then Keep anyway.
In Chrome: click the ··· on the download and choose Keep.
The safe-looking button cancels the download. The one you want is the quieter one.
Open the file from your Downloads.
A blue box says “Windows protected your PC”. Click More info, then Run anyway.
This is the grown-up part. A dialog asks “Do you want to allow this app to make changes?” — it needs the PIN or password. A grown-up types it.
A black window opens and prints a few lines. When it says OK – rivendell is …, it worked. Press a key to close it.
In Minecraft: Multiplayer → Add Server. Put anything you like in the name, and this as the Server Address:
rivendell
Then Done, and join it. It stays in your list from now on.