I work off desktop but need to be able to invoice from an iphone and ipad. where can I find the instructions to do this please?
If you mean "desktop" as in you access QuickBooks Online from a desktop computer, then you can install the mobile apps for QuickBooks Online and have access to your account to send invoices.
If you mean you are using the actual QuickBooks Desktop program, then there is no native way to access your data file from an iPhone or iPad. You would need to use a 3rd party software for remote connection and then remote into your computer from the mobile device and access the QuickBooks Desktop program that way.