Come in with the candle, sit in the chair and hold the candle in your lap. That should be the sole illumination on the stage. Sing the whole aria from there.
Enter with Barberina trailing behind, who's holding the baby. Barberina sits in the chair. When the Countess says "Fammi or cercar da una mia serva aita!", Barberina rolls her eyes. More strongly with every repetition.
For Dove Sono, the Countess takes the Baby from Barberina and sings to the baby like a lullaby. Barberina understands that she's immediately expected to get up and let the Countess sit.
When the aria gets to "Ah! se almen la mia costanza,", the Countess gives the baby back to Barberina and indicates that she should leave the room. The Countess then moves over to the Count's desk and sings the rest of the aira to the Count's chair.