What to do before make full polished game?

Ok, so I have 8 days school break because my school won handball, bowling, archery and football(soccer) national tournament in one day. Little bit weird but yeah... Ok so, I want to create a new, polished game. What to do before doing that really big task(for me...) Maybe there's a thing that I really need to check or maybe there's a thing that really important. I don't know how to say it so... I hope you understood what I just wrote...

Before you make a highly polished game, always make a prototype first. A single level or smaller version of the game, with simple graphics (like quads and triangles and circles instead of real graphics). By that you can test out if the game mechanics work and you know exactly what assets you need later so you don't have to throw out super complicated graphics that took a long time to make, when you find out that the enemy/level or whatever is not needed anymore.