API for Carts on the Terminal

Bryan - Great news.
My specific use case involves needing to create a cart associated with a Square Customer with specific Square Items that the register can then bring that up and add/update items and then accept payment at the register.
I already have the customers integrated to Square, so it’s now a matter of creating a cart.
What are the steps to do that and is the POS API involved at all?
Thanks,
Jeff