In order to do this, both you and your friend will need a program called Hamachi. After installing, one of you creates a network and the other person joins. The host must write down the big numbers on the Hamachi window, that is the I.P, once the portal game is launched the host has to type in `maxplayers 2` in console, followed by `sv_cheats 1`, then `lan_ 0` and finally `map whatever-map you want to play`. When the game starts, you can type impulse 101 in console to get everything.

The other player who isn't the host types `connect "ip"` in the console. OPEN UP CONSOLE AND TYPE IN `Coop 1`.