That is another thing I had problems with when I started doing games. I always thought the best route was to write down every single feature of the game on paper and have a huge design document written out so I knew everything I had to do.
The problem was though that I spent so much time on writing up them design documents so that everything fit together that I never even started on half of the games.
Another problem was that a lot of things sounded like they would work or be really fun on paper but in reality they were way to hard to implement or they were really boring to play.
Another problem was that it if something doesn't work in your design or isn't what you thought it would be and everything else you wrote down on paper depends on that thing that doesn't work you are basically stuck with a bunch of paper that is worthless ;p.
Games projects are very fluid and go through major changes in designs throughout the whole development process. Somethings sound great on paper but suck in reality. So yes it is a good idea to write basic ideas down about your game but remember most of them ideas will probably change during the actual coding of the project so be flexible and don't spend to much time on writing down your whole design.
So since your professor wants you to write thing down just write down basic ideas for your game and that is about it don't go into major detail about every aspect of the game and be prepared to cross out ideas and change them.
thanks Zereo. I know what you mean, I did the same thing before on a different project. I spent way to long on the documents and not enough on the coding. I won't make that mistake again with this project
@OP The reason to create a game on a smart phone is audience and portability of device. Unless you have a tablet with Windows/Linux/Mac you are stuck playing the game at either a laptop or desktop. Phones are a lot easier to take with you and usually always have it on you so you can play the game anywhere and anytime.
Keep them as simple as possible. If they get detailed, I draw them on paper and then scan them in. From those scans I'll create a vector version of them for easy editing. Then I'll just export a png version at whatever size I want.
Also, it's good to batch up your graphics. Reading individual files for each sprite is inefficient.