You can create a single invoice by clicking on the '+' icon on the top header.
Select the name of the customer and the date range for the Single Invoice. This will show all the appointments that have not been Invoiced yet
You can select all the line items or choose the appointments that you want to invoice for.
You can edit price of any line item by clicking on the Edit button on the right which will bring up the edit price form. Enter the new price and click submit.
Custom Line Item
You can add custom lines to the invoice which can be additional surcharge or discounts for the Invoice.
After selecting the line items that you want to invoice, click on Tax and Discount to see what the final invoice balance will be.
You can also add a note at the bottom of the invoice. Select buttons at the bottom of the page; if you want to create the Invoice in Draft Mode (can be edited later) or Finalized the Invoice.