There are two ways to go about taking advantage of this feature, both of which are involved with one specific setting in your Company Preferences labeled "Allow Invoicing Prior To Job Completion".
If the above toggle is set to No, then you will need to Close Jobs Without Invoicing them. This will put your Jobs in to a Jobs Closed status to be invoiced at a later time.
If the above toggle is set to Yes, then any Job that is not invoiced will be able to be combined in to one invoice. Please note: When the toggle above is set to Yes, a new option will appear that will give the option to automatically create invoices. If that toggle is set to Yes (to create invoice automatically) then you will not be able to combine multiple Jobs on to the same Invoice.
To create an invoice comprised of multiple Jobs, hover your mouse over the Accounting tab and select Create Invoices.
On this Dashboard will be a list of all Jobs that were selected to be invoiced later and they will be organized by customer. Please note: Only jobs for the same customer can be combined on to one invoice.
Find the customer that will the invoice is being created for and select the check boxes to the left of the Jobs that will be included on the created Invoice. Once selected, click the green "Create Invoice For Selected Job(s)" to create an invoice from the selected Jobs.
The created Invoice will list each job that was added to the Invoice with the accumulated total along with all other Invoice information that would normally be present!