Get the Essentials
Get Super Smash Bros. Melee. You can play on console (GameCube/Wii) or emulator (PC). To work with Slippi, make sure it's a NTSC v1.02 (USA region). Also obtain an official GameCube controller. Make sure to get first party, Nintendo-made controllers only (advanced guide). Rip your copy of the game and save it as an ISO for use with Slippi.