How to create a new sales invoice using the SDK?

I am attempting to write a program to transfer information from our sales invoices in one system into our sage without manual typing the information. I've been trying to use the SDK and though I seem to have found methods for creating new clients or other services I cannot find the methods or classes for creating a new invoice. I am using the .Net C# code. Currently I am trying to use the SalesJournal: SalesJournal invoice = SDKInstanceManager.Instance.OpenSalesJournal(); But I still seem to be missing something to get it working. Any help would be greatly appreciated!