Go to Nexus to download the LAN multiplayer mod or use this ‘Co-op Helper Tool’ in LagoFast!
SlayTheSpire2.LAN.Multiplayer at Slay The Spire II Nexus - mods and community

When you launch the game after installing the mod for the first time, a warning will appear. Select "Load Mods" and then restart the game.

Select Multiplayer
Select "LAN Host" and wait for other players to join.

Select Multiplayer

Select Join

"LAN IP" field, then click Join to enter the room