From Sales → In Invoice AR List click "Create Consolidated Invoice"
This will bring up your Create Consolidated Invoice window
Date: This field is the date that the new consolidated Invoice will reflect as when it was created
Customer: You will select the customer whose invoices you are trying to consolidate
Terms: Terms of the new Invoice you are generating
Once you have filled the required fields, select the invoices you would like to consolidate and click Apply.
All consolidated invoices will end with a C, for example INVC-1.
You can also view all consolidated invoices by their status in the Invoice AR List.
In order to UN-consolidate invoices, make sure you are looking at the top level invoice and not one of the invoices contained in the consolidation. To do this select Consolidated from the Type filter to ensure that you are only seeing the Consolidated invoices.