NOTE: this install is for version 1.20.1 – Old versions here.
1. First, make sure you have the latest version of the Java JDK installed:

2. Next, download MultiMC for free online:

3. Now, go ahead and launch MultiMC, if you may get a warning – make sure to permit it:
4. After this you’ll need to add the WHIMC instance, add it like this:
Instructor (teacher) instance:
https://uofi.box.com/shared/static/fwfdkxyymb7vpefu6fo8p4rktzsjypwn.zip
Participant (kid) instance (includes celestial bodies single player maps):
https://uofi.box.com/shared/static/76t6ystxthxtc5zxir89gb977ocdwbeb.zip
Dome Theater (Celestial Bodies) instance:
https://uofi.box.com/shared/static/dbcz3tgeovhmd9kew490wqt2ao5g21g8.zip

5. Finally, login with your Minecraft Java Edition account:
IMPORTANT: If you get a java error tell it to auto-detect Java under settings.
You’re good to go, launch the instance!