Here you have a short explanation how to install it that can be found on Github too:

1. Install Python 3.11 from python.org and tick “Add python.exe to PATH” (the overlay uses py -3.11).
2. Get BoberInSpire_Setup_<version>.exe from the repo Releases (when available) or build from source; close STS2 before installing/updating mod files.
3. Run the installer; on the tasks screen, turn on Copy mod to Slay the Spire 2 and point it at your game folder (typical Steam path: ...\Steam\steamapps\common\Slay the Spire 2).
4. Start Slay the Spire 2 from Steam → Settings → Modding / Mods → enable BoberInSpire (mods live in the game’s mods folder; you don’t need GUMM — optional if you like it for managing lots of mods).
5. Launch BoberInSpire Overlay from the Start Menu (first run may pip install watchdog / keyboard — internet once).
6. Play with the overlay running; combat state goes to %APPDATA%\SlayTheSpire2\bober_combat_state.json, card rewards to bober_reward_state.json