Instructions
Get a baby male and a baby female monkey in the game.
Raise the male and female monkeys so that they become fully mature adults; they cannot mate until they are both adults.
Interact with and play with both monkeys in the same room. Give them toys to play with so that they can play with you and with each other; this will let the monkeys start developing a relationship.
Determine your monkeys' compatibility with each other. If there is a smiley face between the two of them when they are together, then the monkeys like each other and will likely mate.
Put the monkeys together, alone, in a room. If they like each other, they will mate and have a baby.