When you click on "Accounting & Finance" tab, there will be "Add New" drop-down menu on the top left corner of the screen. Please select the "Sales Invoice" from "Add New" menu and you will be directed to the new blank sales invoice form. You can either copy from the existing sales invoice or create new sales invoice from a blank template.
You will be asked to fill the following required fields:

Client
Address
Currency
Invoice Date
Due Date

Once you've filled those required fields, you are free to use other optional fields, such as Shipping Method, Paypal Recurrence Settings, make them tax exclusive or give them reference, etc.
Also, you will be asked to add the Product/Service, where you can give a description and set a discount, tax rates.
You can add or remove multiple products and service by clicking "+" or "-" sign.
Additionally, at the bottom of the line, there is a "Add multiple lines" button where you can perform the same operation.
You are free to write down or copy and paste the Payment Instructions and Notes in " Payment Instructions and Notes " text box. Or you can set your own template from accounting settings. You can specify bank details and add a bank account for the payment from "Bank Details" section.
Great! Sales Invoice is Ready!
Now you can either download the .pdf version by clicking "PDF Version" button or approve and save it.
If you click on "Approve and Send to Client" button, system will bring up the pop-up window from where you will be able to send an email notification about the sales invoice. It will automatically include the PDF version of the file in it.