Make sure to enable the dev console in the Advanced section of the game's controls, or go into the launch options and in the textbox, type "-console". This allows you to move on to the next step.
Press the key underneath Escape (`/~) to open the console. Type in "map background02" in the console and press the Enter key.
Type in the console "ent_remove info_target". This allows you to move your character.
Type in the console "give weapon_portalgun" and "upgrade_portalgun". This step is optional.