After selecting a room for the night activity, you can choose one or more skills (up to 7) from the maid's repertoire depending on what is compatible with the room. Certain skills (when unlocked) will only be available in certain rooms, such as public (Courtyard, Outdoors), sharing (Stage, Penthouse) or SM stages. Each skill comes with 3 levels. As your maid gains EXP and improves the skill, the stats gained and lost per action are improved.
Depending on the maid's Stamina stat (bottom left) the amount of skills you can equip for the night without dropping to 0 or less Stamina is limited. If you go below 0 Stamina, your maid will fall unconscious at the end and fail the next day's work, which can be useful if you want to punish her the next day (Option only available after failed day work).