It's a two step process. The first one is to get at least one of the candidates to recognize the other (relations are not always mutual). I have no idea how to promote that apart from time. Newly grown adults rarely, if ever, know any of the other kids they grew up with.
Improving familiarity with others is done by socializing, which is where the second step comes in.
The second step is a pre honeymoon suite:
A room (I've used 3*3 to good effect) with a food and a booze stockpile, plus beds for the two candidates (remove their previous bed ownership).
Set up a tavern zone (can be part of the normal one) and burrow the two candidates there. It can take an eternity for them to actually get and stay there, since the needs system causes burrow violations, though, so a door to lock them in once they get there works, as the incorrect MillBuilding task eventually gets replaced by the correct MillingBurrow.
Once the two are in there alone, lock the door (probably not needed, but the fewer external parties around the more focus they'll presumably have on each other.
If they're compatible (at least one recognizing the other, within the 10 year age range, willing to marry the other party's gender, and without disgust at the thought of marriage, etc. I haven't tried any couple of related dorfs, although I've heard it said only siblings are prohibited [parents are automatically excluded both on the age restriction and the fact that they're {usually} married already]), they'll gradually increase their familiarity and become lovers and then married. Once that happens you'll remove the tavern zone to let them spend time on no job. You can check up on the female with DFHacks gui/gm-editor relations/pregnancy_genes. Once that changes from a nil value she's pregnant, the couple can be let out, the beds reassigned to the next couple, the stocks can be topped up. Rinse and repeat.
You can repeat the last step in their room once a dorflet has been produced to keep them coming. Be prepared for frustration with their burrow violations, though.
Edit:
And concerning pisskop's advice: I've heard it before, but my experience is that militia members are extremely poor at getting to recognize other dorfs. I had a case where it took several years for a drafted migrant to recognize anyone, and that was the squad leader. The other squad members remained complete strangers for several years more...