Instructions
Ensure you have both the "Archangel" persona and the "Berith" persona in your Persona deck, then enter the Velvet Room. The door to this room can be found either in the bottom-right corner of the TV world, or near the middle of Main Street in Inaba.
Select "Fuse Personas" when Margaret asks you what you wish to do, then select "2-Persona Fusion" from the option list.
Select "Archangel" as your first persona.
Select "Berith" as your second persona.
Confirm your choice. Igor will fuse your Archangel and Berith personas into the new Sarasvati persona.