Question: how to install rl minecraft?
Answer: Download forge-1.12.2-14.23.5.2838-universal.jar from here.
Install forge (select install client)
Download RLCraft Server Pack from CurseForge.
Go to %appdata% folder.
Create a new folder in it and rename it to rlcraft.
Extract server pack archive to rlcraft folder.
Run minecraft launcher.