1. Make sure that you have a more or less equal amount of females and males on embark.
2. just one meeting zones, 1x1 size (as Retro said in the posting above).
3. they won't need a massive lot of free time, but they need chances to talk at the meeting zone. Just don't keep all of them busy all the time up to the first autum, and the relationships should have started. The initial 7 dwarves and the odd migrant that appears in the first migration wave are very eager to form relationships. Much more eager than the 2nd generation will be.
Sometimes, the last couple of original dwarves will refuse to fall in love with each other. If that happens, you're out of luck, and not even locking them up in a tiny room together will help. I don't know what causes this, maybe some of these are gay.
Another serious limitation to these inbred fortresses is that they generally don't want to form relationships when the age gap is "too big". Little Rakust, firstborn of my last fortress, was apparently considered "too old" by the girls that were only 2+ years younger than him; and nothing happened between him and the two "old ladies" from the first generation either.
.. if only the FPS bomb hadn't hit that fortress. I would love to play it into the next generation.