The skins you import must be one of these categories: character base, item.
The animation file must be using zip format instead of dyn.
Put animation assets in anim/.
Put inventory item atlas and image correctly.
Put everything else correctly.
Load them yourself. Typically you load ANIM, IMAGE, ATLAS and ATLAS_BUILD.
Call RegisterInventoryItemAtlas as in DST. You may still need to hash.
Call CreatePrefabSkin() as in DST.
You don't need to specify assets because I will not load it for you.
You don't need a init_fn as what Glassic API requires.
You need to write SKINFNS.xxx_init_fn and SKINFNS.xxx_clear_fn instead of putting them in global scope.
If your skin has fx, put it in SKINFXFNS. Also you may combine it into SKINFNS, but remember when a placer is spawned, SKINFXFNS will not be touched.
However, unlike DST, it is OK not to write any init_fn, it will naturally fall back to basic_init_fn.