First, select the target track in your library. I wanted to generate a playlist for work, so I selected a mellow track by Band of Horses. Next, click the Genius button (with the icon) in the lower right-hand corner of the iTunes window.
Presto! Your playlist is created. But there's still room for tweaking. At the top of the window, you'll see the track upon which your new playlist is based. Next to that, a drop-down menu lets you select the number of songs to include. Choose from 25, 50, 75 or 100.
Finally, you can exclude an individual song by deselecting the gray checkbox next to its title. When you're happy with the result, click "Save Playlist" in the upper right-hand corner.
Your new playlist will now appear with the others, named for the original target song.