Disclaimer: Playing backups of games you do not own is illegal. This guide is for educational purposes and for playing homebrew or backed-up media.
After the game is successfully installed, you connect your USB drive to your PS2 Slim. You then boot your console using the USB Extreme program. From its menu, you highlight your game's name, select it, and choose "Start" to load it. Download Usb Extreme Ps2 Slim