Follow along on your board if you like. I'm exceptionally proud of this one, especially being on the hardest level of computer here at ELO 2400. I played Black.
1. e4 d5 2. exd5 Qxd5 3. Nc3 Qe6+ 4. Be2 Qg6 5. Nd5 Qd6 6. Ne3 e5 7. Nf3 e4 8. Ng5 Qe5 9. h4 h6 10. Nh3 Nc6 11. d3 exd3 12. Qxd3 Nf6
It seems so far that White has pretty good development, but it hasn't managed to castle yet and get out of the line of fire. I'm about to make it sorry it didn't.
13. Nc4 Qe7 14. Nf4 Nb4 15. Qb3 Qe4 16. Na3 Bd6
Even more pressure on the knight, forcing White counterplay. I'm not quite sure about f3 here at all. Seems very weak.
17. f3 Qe7
Chases my Queen back, but she is just biding her time now.
18. Bd2 Bxf4
The first major exchange and already White is at the disadvantage. This capture was merely to clear some space.
19. Bxf4 Nbd5 20. Qb5+ c6
Questionable. Perhaps it's to cover the frozen e2 bishop.
21. Qa4 O-O
Finally I can castle. White, as you can see, still cannot. And it is getting dangerous.
22. Rh2 Nxf4
Another clearing capture.
23. Qxf4 Nh5 24. Qe4
White wants to trade queens? Not when I have a much better move:
Threatens Qg1+ and winning the rook. At this point, I cannot imagine why White didn't castle here: O-O-O seemed good.
Threatening mate. But I had Nf6 to play - and it did something a LOT more than just stop mate and threaten the queen. Can you see it? Note the wide open d and e-files. This spells disaster for White shortly.
I'm salivating at this point.
26. Qf4 Qg1+
I play the killing move. This essentially sealed the game. Again, White's failure to castle was key here.
27. Kd2 Qf2+
Not content with taking the a-rook. I smell blood.
Almost forced. If Be2, then I play Re1+ and it's a wash for White.
The end. White is about to be forced to give up his queen.
What better move did White have but to threaten mate? Sadly I also threaten it with one more move.
Finished. White has NO choice but to take the knight with the queen. The only other option is Kc1, which leads to a swift mate: Qe1++.
30. Qxe3 Qxe3 31. Rh1
Trying in vain to get my queen into some challenges with Re1. I have other plans.
32. Re1 Rxd3+
33. cxd3 Qxd3+ 34. Kc1 Bf5 35. Re4 Rd8
And from here, you can see a forced mate in 3-4 moves.
I hope you enjoyed this instruction, and that it helps you. Please examine this game on your own, and see if you can spot where White went wrong. Enjoy!