The most important is to know how to launch it!
Use steam option to add starting parameters and write there "Lolailo" no minus prefixes etc.
If game wouldn't show you the starting resolution box, where you had additional Developer tab, don't worry you will have dev keys in game

Now as you are the developer of this GAME OF THE YEAR.
You have dev keys enabled.
Few examples

Pressing "1" gets you: GAME ENDING VIDEO

Pressing "F5" gets you: Free roam camera that uses LMB and RMB to move forward and backwards.
Protip: You can still move your char, but it's movement is still fixed by mouse and camera.

Pressing "F9" gets you: Teleports you to next known position a.k.a. Save Points or important switches
Protip: It can help you speed run to next level.

Pressing "F11/F12" gets you: Switches your vision to polygon meshes without textures a.k.a. "Nie po raz pierwszy stają mi polygony i shadery" option.