Invoices(s) can only be created after the sales order is confirmed.
To start invoicing the sales order, go to the invoice tab in the sales order or click Invoice.
Two ways to Invoice:
To partially invoice the sales order, select parts of the sales order to invoice e.g. some line items only or lesser quantity etc and then click Confirm.
To completely invoice the sales order, just click Confirm and it will automatically create a invoice with all the uninvoiced items in this sales order.
Editing a invoice
If you wish to edit the invoice you created, simply find the invoice and click on 'Edit' to make any changes to the invoice and click save.