They're in various threads, as the issue crops up fairly regularly...
Marriage:
- Check that they're willing to marry opposite gendered partners (I'm assuming you're aiming for children here). Dwarf Therapist provides great support in this department. Also check personality traits, as those who hate the thought of a family and never have felt a connection to any other living being are probably not good candidates.
- Make sure at least one knows the other (it's not always reciprocal!), and, obviously that it's not a grudge. This might not be required, but it's what I use.
- Make sure standard conditions apply, i.e. within age range, not siblings, and neither has been married before (and same race, for the [probably rather long] time being).
- Make an overlapping bedroom with one bed for each of them.
- Make a food and a booze stockpile of two tiles each (one is going to be used by an empty barrel) and make sure there's food and booze in them.
- Make a door to the room.
- Pull both subject into their bedroom by using a burrow covering their bedroom. I reuse that burrow when it's time for more children by adding them to the burrow again. I usually assist the process by painting the bedroom as a tavern and allocate this zone as a part of the fortress tavern. Note that cancellation spam can happen when you try to pull them in. Remove the spamming one from the burrow and readd him when the current hauling job is finished (before something else has been picked up). Also note that the buggers can refuse to cooperate and socialize/pray in the big facilities instead, and they can even temporarily make brakes from these to eat/drink/sleep in the bedroom and then return to the external activity.
- Once both of them have been pulled in, lock the door. If you're reasonably sure they're not going to desert and you've got freeloaders in the bedroom (on account of it being a tavern zone), remove the zone and open the door, and lock it again when they're alone. If you don't lock the door you're going to get more freeloaders and also cancellation spam when one of the subjects decides to top up one of the stockpiles, but repeatedly failing due to the burrow.
- Once they're locked in and alone, repaint the tavern zone and have them socialize until they're married, which usually takes less than a season. It can be successful after a longer time, but there's probably a stumble block somewhere.
- Once they're married, remove the tavern zone. Dorfs behave a bit like humans when the power goes: the only entertainment to be had is with each other... Usually the female is pregnant once they've both been idling in adjacent (or the same) tile and one of them moves, but stubborn ones have been observed to require multiple opportunities (I use a script of my own to check for pregnancy, but gui/gm-editor can also be used, or you may just give them a month, which should be more than enough).
Pregnancy:
- Pull the couple into their bedroom with a tavern zone (as above), get freeloaders out, lock the door, and remove the zone. Time for them to get down to baby making again...