The title explains itself. I have looked at the guide in Guides, but nothing appears to be happening. What are the exact actions I have to do, and are there any things I should pay attention to?

So you press tab, and a yellow frame should appear around the enter race button. Then click the right arrow key at least 6 times, and the yellow frame should disappear to the right of the screen(u can press right more to be on the safe side), and then hold space down. I hope this helps.

I assume you mean the level upgrades? You just have to beat the game and use the cheat options. They will carry over to a new save.

If you mean the glitches to gain levels quickly within the minigames, those can be performed with the right click functions, allowing you to go forwards/backwards through scenes without pressing buttons. These can be used with Alt+C +F/B depending on if you want to go Foward or Backward. Watch the record to figure out when to go forward/backward to gain levels.

Otherwise, if you mean the dev "level +1" button that's located off screen, refer to @Rhino97 's response.