Inspired by the work of Calen Henry, I set out to do my first NES ROM hack: a playable version of Minimal Mario. The end result is awesome, and playable.
First, you need to make a dump of your Super Mario Bros. cart. The cart I had ended up being detected as "Super Mario Bros. (JU) (PRG0) [!]", in the GoodROM database. You'll need to make yourself a dump of this same cart. I recommend using CopyNES from Kevin Horton. Unfortunately, he's stopping making them for others. You can buy a newer USB version of the CopyNES from Retrozone. Refer to the CopyNES software documentation on how to make a clean dump of the game.
The next step is to apply my IPS patch to your freshly dumped game. To do this, you'll need an IPS patcher; pick your favorite one. (Hint: For Windows XP, I like Lunar IPS. For Mac OS X, I like UIPS. For Linux, I like to use whatever the default package manager provides.) Apply the patch and you are ready to play.
When you first start the game, you'll see this screen. To start a single player game, press start. To start a two player game, press select, then start. This is exactly the same as the normal Super Mario Bros. game.
A fellow named Billy, who found that the springboard wasn't very minimal in my original patch, has uploaded a video of a play-through to YouTube. Be sure to watch it in all its glory in super low resolution 240p!