The milking minigame is available once Diane’s and Daisy’s routes have advanced enough, so after the barn is built, and if the relationship with the partner is established. It can be played again and makes the time of day pass.
Press A and D, ← and →, or tap on left and right sides of the screen. Press each side, in an alternating pattern, until the milk reaches beyond the purple lines and both machines are full. You cannot fill one side at a time and the minigame fails if one breast is milked several times in a row.
The profit ranges from $100 to $200, the gain varying according to the pregnancy status of the person milked.