{ "GUID": "your-item-guid", "name": "Custom Underwear", "description": "A custom underwear item." }

You can only have one default replacement for each outfit category/age/gender at a time. Installing two will cause conflicts (texture glitches or Sims turning invisible).

: Sims can also change their underwear in their wardrobe. Click on a Sim, choose 'Wardrobe', and then you can select to change their underwear if they have more than one set.