1. Navigate to the Invoices menu and chose the 'add new invoice' option on the left side of the screen 

2.  In the 'Add new invoice' menu, click on the button next to the 'Existing customer' field and chose the customer you want to add the invoice for 

after the customer is chosen, check if all the required fields are filled up automatically

3. Navigate to the bottom of the page, there you have 3 ways to create an invoice:

    3.1 - the product menu : This menu allows you to specify a product that will be billed in the invoice,.

    Click on the 'select product' and chose the product you want added from the drop-down menu, and if you want to add           additional products, click on the 'add new product' button

    3.2 - importing invoice lines from other invoice 

     This option allows you to automatically import the products from an invoice you want recreated,

     just add the invoice reference number of the invoice you want to be imported, and the products will be automatically             added to the  product menu

    3.3 - adding invoice line from subscription

    This menu lists all the subscriptions of the chosen account, and you can add the invoice line for it by clicking on the 'add'        button in the 'id' column 

4. after adding the invoice lines you want, click on the save button on the bottom right part of the page, an invoice will be generated automatically