There are four ways to do this:
Option 1. Go to Sales → Sales Transactions, click Plus and select Invoice.
Once you've chosen the customer, you will see a side panel with a list
of all open sales orders (if the customer has any). Mark one or multiple
orders and click Add, or skip this step to create an invoice without sales orders. When you are finished with the invoice, click Save.
Option 2. Go to Sales → Sales Transactions and find the sales order you would like to invoice. Select Create Invoice in the Actions column. On the New Invoice page you will see a side panel with a list of other open orders (if this customer has any more). You can mark them and click Add to include them on the invoice, or skip this step to create an invoice without sales orders. When you are finished with the invoice, click Save.
Option 3. Click Create and select Invoice under Sales. Once you've chosen the customer, you will see a side panel with a list of all open sales orders (if the customer has any). You can mark one or multiple orders and click Add to include them on the invoice, or skip this step to create an invoice without sales orders. When you are finished with the invoice, click Save.
Option 4. Go to Sales → Customers and find the customer you want to invoice. In the Actions column (the last column), click the drop-down menu and select Create Invoice.