I don't know multiplayer very well but this should get you started:

- first look up your computer's local IP address. this starts with 192.168. e.g. https://lifehacker.com/5833108/how-to-find-your-local-and-external-ip-address
- Use the "Multiplayer" section of the main menu in Quakespasm to start a game on your computer. Your player should be in the game.
- On your friend's computer, start Quake and go to the console. type "connect 192.168.XX.YY" replacing the number with your computer's local IP. It should connect and have both players in game.