Use a clean save and play through levels 1-30 without closing out of the game. Your time is then recorded in the Gallery, the time which you should submit.You can reset game data from the gallery, or use an in-private browser if you wish to keep your saves. Playing the game on a different site (Newgrounds, Kongregate) can also start you with a new save file, provided you haven't made progress there already.
Take note of the scores you achieve in Music for Decay and submit the highest one at the end of Day 1. If you use a clean save, it will automatically record your highest score, indicated by the small, darker numbers below the current score.
You'll have to be careful here because the current Scores only show for a brief moment as the character is dying. Since it is a turnbased game, however, you can just record your scores just before inevitable death. If you use a fresh save, you can grab the "Best" score from the top of the screen at any time.