Click to install the RhinoHair plugin: RhinoHair (website)
Open the zip file. It’s probably in your Downloads folder. Extract the file to someplace you know where it is (Desktop).
Right click on the file > Properties > Unblock. This unblocks the file because it came from the internet, and Rhino doesn’t like blocked files.
Drag the file (rhinohair.rhp) into an open Rhino session. It should say “Drag and drop loaded plug-in to [file location]”.
Good job. The (only) command to use the plugin is “RhinoHair”.
For reference: 50,000 hairs, length=1, segments=3, sides=3, other settings=default; takes 65 minutes on the lab computers. The program goes through 3 iterations per hair, so 1,000 hairs requires 3,000 iterations. Every 1,000 iterations generally takes 20 seconds, so you can roughly calculate how long it takes to grow your own hairs.