Texture not displaying on a player

My friend and I are creating a datapack and resource pack, but the problem is that we’ve managed to make the item display the correct texture in hand and in the inventory, but it doesn’t show correctly when worn. We know that we need to use layers, but we don’t know how to set everything up, because we don’t want to replace the base armor files. Instead, we want to use CustomModelData so that all the default armors still exist, along with the ones we’re creating.