Once they've fallen inlove (in other words, have 4 heats) wait 24 hours and then they should procreate.
The Father will go to the mothers screen like as if it were gift giving, you'll see the screen go dark and then fireworks.
Once the father has returned to his screen the mother will have two little babies with her on her screen and one of them will lift off and lower onto the fathers screen. You're now free to carry on your tamagotchi care as per usual, with a tiny Tama attached.