Lovely Injector: https://github.com/ethangreen-dev/lovely-injector
This video is slightly outdated, you don't need smods-old-calc anymore, this one is good:
Steamodded: https://github.com/Steamodded/smods/archive/refs/heads/main.zip
Balatro Multiplayer Discord: https://discord.gg/balatromp
0:00 Intro
0:40 Installing Files
3:23 Steamodded
5:08 Multiplayer Mod
9:22 Running your own server