On the Partner portal, click on the "Customer Invoices" (ref. 1.) tab, then the "Add" (ref. 3.) button to bring up the invoice builder.

Specify the customer (ref. 1.) and the due date (for payment) (ref. 2.) for this invoice.

Finally, select and drag across (ref. 3.) to the left desired products. Click "Create" (ref. 4.) when you are done building the invoice.