How do I submit my order using in-house billing?

Within your back office, Orders > Add Order > This Order is for myself page, you can "add to cart" what it is you want to purchase.

On that same page underneath your cart selection, you will see two options, Continue Without Discount or Apply One-Time Discount. The Continue Without Discount button will put the order into billing. The Apply One-Time Discount button will continue the order as a cash type order.