Hi
@nitaxim - I know this reply is a bit late, but the “default rules” for domestic single-member LLCs are that it is “disregarded” and treated as part of the owner’s tax return.
All of the income and expenses would be combined with the parent C Corporation. Additionally, you would just list the entity and its EIN as part of the listing of entities the Corporation owns.
Other than that, I don’t think there is generally much else to do. Of course - don’t forget the franchise tax return or annual report required for the LLC (unless part of a consolidated filing).