Foci can be handy for any Awakened character. To start, open the Add Gear window and select the Focus and desired Force.
Simply adding the Focus isn't enough. The Focus must be Bonded to the character in order for them to gain its bonuses. If your character has access to the MAG Attribute, your Gear tab should also include a Bonded Foci section for managing them. Find the newly-added Focus in the list and place a checkmark next to it. This will consume the necessary amount of BP or Karma for the Focus and add its bonuses to the character. You can also break the Bond with a Focus by removing the checkmark.
The Street Magic sourcebook introduced Stacked Foci which lets you combine multiple Foci into a single entity, effectively increasing the number of Bonded Foci a character can have at once. In order to create a Stacked Focus, the character must have two or more Foci that are not currently Bonded. Click the Created Stacked Focus button next to the Bonded Foci list to start creating a Bonded Focus.
Choose a Focus from the list in the window to add the Focus to the new Stacked Focus. This window will keep appearing until you no longer have any eligible Foci to stack or the Cancel button is clicked, in which case the Stacked Focus is created from the Foci that were added up until that point.
The selected Foci are removed from your character's Gear list and are replaced by a single Stacked Focus item. A Stacked Focus will also appear in the Bonded Foci list and can be Bonded in the same manner as described above. The cost for Bonding a Stacked Focus is the same as though you had Bonded each Focus individually. The only benefit of Stacked Foci is reducing the number of Bonded Focus the character has at once.