See the video below for a walkthrough on creating sales invoices:

1. Go to Sales.
2. Select Sales Invoices.
3. Select New Invoice.
4. Choose the Customer, then fill the remaining details; Invoice Date, Description, Ledger Account, Price/Rate, VAT Rate etc.
5. Confirm the details are correct.
6. Select Save.

For a more detailed breakdown of the details on the invoice, read create a sales invoice- detailed. 

Create a purchase invoice