You define the item slot in your .yaml file by specifying the root entry type via a $base record. Base records come in many different types, some include intrinsic modifiers that add slight stat improvements to items, or add special sound to items. A complete page with clothing items, their intrinsic modifier, and their baseids can be found on the official Cyberpunk 2077 wiki's article for clothing here.
Items.My_Custom_Shirt:<< name of your item (the spawn code)$base:Items.ShirtItems.My_Custom_Helmet:$base:Items.Helmet# For items that should hide hair, use records with the "Hair" suffix.Items.My_Custom_HelmetWithHair:$base:Items.HelmetHair# For items that should have an intrinsic modifier, e.g. ArmorItems.My_Custom_HelmetWithArmor:$base:Items.Helmet_Intrinsic# For feet items that use heels (clicky sound)Items.MyCustom_Heels:$base:Items.FormalShoes
You can absolutely set up a shirt and then put $base: Items.GenericHeadClothing - V is the only person I know of who can wear their undies on their head without impairing their functionality.
However, before you start abusing the system that way, you might want to look into EquipmentEx.
Step 1.5: EquipmentEx
psiberx's mod EquipmentEx (github | nexus) adds a whole new wardrobe system, providing extra slots that CDPR forgot to include with the basegame. This feature requires the mod to be installed — without it, only the $base types from step 1 will be considered.
All you need to do is adding the last two lines to your .yaml:
Find a full list of outfit slots in the github readme.
Step 2: the entity file
Some item properties are defined in the file mesh_entity.ent via components. If you use the wrong kind of entity, you might end up with your shirt being a puddle around your feet, or string cheese. To get around that, you need to make sure to pick a file that correspond's to your item's body part.
You can find all entity files under base\characters\garment\player_equipment:
Pick any file from inside the correct folder.
If you want to be super thorough, you can stick to the right gender
If you want to know more about components, check the wiki page. This is completely unnecessary for the purpose of this guide!
(Optional) Step 3: Hide it in First Person
You can hide items most easily via ArchiveXL tag. Check the linked page for how to do that.
Setting footwear sounds
If you don't want your new boots to sound as if V was barefoot, add one of the following tags to the Root entity: