Take Art

You'll notice I haven't said anything about pictures (or music) yet. Well, I'm not going to do these myself and I shall be throwing myself onto the mercy of some artist and musician friends later, but how do we get by for now? We use placeholders, and there are two traditional ways to get them.

1) Theft
2) Programmers

Both have the advantage of speed, so we don't waste too much effort on something we aren't ultimately going to use.

Theft
Well, piracy. Simply use somebody else's graphics (sounds, fonts) from someone else's game, or the Internet. Very quick, and you get good quality results straight away. You can get away with it, as nobody will ever see (hear, read) these 'borrowed' assets. There is one major drawback, however.

You will forget.

You will forget to remove something from the final game, and then you can be in hilarious* legal trouble with your finished game. Not good. For this reason, I'm completely going to avoid this route and go with...

Programmers
Programmer art! It's great! Or not! Yes, programmers can do pretty much anything as long as all you want are some correctly formatted data files. We're spending no time here, so doubly bad. Programmer assets do have a big advantage though: they're noticeable. You aren't going to accidentally leave your enemy marines as bright pink rectangles. With any luck, they'll be so bad that artists will replace them on sight. Bonus.

*Not hilarious.