NOTE: this install is for version 1.19.3 – 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/s/8stgvnejyfib3qycbkx967u4g1vcf0mg
Participant (kid) instance (includes celestial bodies single player maps):
https://uofi.box.com/s/l91xrelzc76yio8b61i8vvvl2qkv9whs

5. Finally, login with your Minecraft Java Edition account:
You’re good to go, launch the instance!