1. Create a shortcut to the Portal 2 exe file. You can also modify the original one.
2. Right Click on the shortcut and select 'properties'
3. Under the 'shortcut' tab add "-console" to the 'target' text box
It should look something like this: "D:\Games\Portal 2\portal2.exe" -console
4. click ok and run Portal 2 via the shortcut you just made
5. Once in-game you should see a console. To manualy load a level, type 'map <map-name>' into the console