Hi,
You can do it either from the frontend or the backend.
Int he backend, you first want to create the user account via the Joomla menu manager, then click on the "new" button of the orders listing, change the user of the order, enter his address, additional information (shipping, payment methods, etc) and add the products with their quantity. Then, you can use either the order status change, or the email button at the top to email the customer.
In the frontend, you would do it like a normal customer: add the products to the cart, checkout, and when you reach the payment gateway, don't pay (or do if you have the credit card information on hand). The customer will receive automatically the emails and if you don't pay for him, he will have a payment link on the order creation email so that he can do it on his own (based on how you configure your shop).
I think that the easiest would be the frontend. However, it will be more flexible in the backend. For example, in the backend you can enter any price you want for the products, change the shipping fees, etc if you don't want the default values that other customers have on the frontend.