I've actually done a fair bit of this with 0.42.X (turned a dead civ from 17 void dwarves into a 140+ metropolis over 70+ years).
- The candidates have to be within 10 years of age.
- Both candidates have to be willing to marry the opposite sex. DFHack's gaydar or gui/gm-editor status/current soul/orientation flags can show this (DT doesn't provide sufficient info).
- The candidates should not find love abhorrent or have a hard time to fall in love.
- At least one candidate has to have some prior knowledge of the other (relations are not symmetric, so one can be a Friend of another who doesn't even recognize the beard of the other). Obviously they shouldn't hold a grudge against each other (I haven't tried that though).
- Prepare a room with:
- A cooked meal stockpile.
- A booze stockpile (this one has to be at least two tiles, as you get an empty barrel in one of the tiles).
- Two beds assigned to each of the candidates (and remove any other beds allocated to them, if any).
- A door that can be locked.
- I prefer a table and a chair, but that's not necessary.
- Set up a burrow in the room. It's a pain to actually get them there, as the new needs jobs are buggy in that they can go to the burrow, eat, and then desert it to pray or whatever. Note that they won't desert if they've realized they're supposed to be burrowed (gui/gm-editor shows mill burrow).
- Set up a tavern zone in the room. A dropped cup is nice as well.
- Wait until they get there, and then lock them in when nobody else is there, to keep them undisturbed.
- Check back once in a while to ensure they aren't thirsty or hungry.
- Socializing like this should eventually get them to marry (within a year in all my successful cases).
- When married, remove the tavern zone to get them to be together without any jobs, which should eventually result in a pregnancy (gui/gm-editor relations shows if the female is pregnant).
- Release, remove the bedroom designations, assign a new shared bedroom for the couple.
- Take a new candidate pair in, possibly after restocking the food and booze stocks.
- Prepare the couple's bedroom in the same way, but without a tavern zone. When a baby has been produced, they can then be burrowed there to encourage the production of further offspring.
- I've tried to have a temple as well, but that didn't seem to do much good, mostly taking time from socializing.
- Otherwise good couple candidates that didn't recognize each other beards (why should you be expected to recognize the 20 dorfs you grew up with, let alone other pesky kids?) were total failures. Socializing in isolation for a year didn't get them to recognize each other (I think I made 3 attempts). I have no idea of how to get past that initial step, other than just let time pass and hope they start to get at least the lowest level of relation.