Looks like you'll need a relationship for that.
Sum ( RelatedTableOccurrence::FieldName ) will compute the sum of all related values stored in FieldName.
If all your records for your "Contract Cumulative Invoices" total have a value in common such as a contract ID or some such. You can use that value in a self join relationship to link to the needed records.
InvoicesTable::ContractID = InvoicesSameContract::ContractID
To create InvoicesSameContract in your relationship graph, you click InvoicesTable to select it and then create the button with two green plus signs to create a new table Occurrence of the same table and then you rename it from InvoicesTable 2 to InvoicesSameContract.
Now Sum ( InvoicesSameContract::InvoiceTotal ) will compute the total values for all invoices of the same contractID.
Is there a place on the FM website that provides step by step instructions for this. I'm fairly new to filemaker and need more detail.
Not that I know of. If I had a more complete picture of how you've set your system up, I could provide a more specific set of instructions. Feel free to post back to here with questions and I'll take it step by step with you.
I've upload the actual filemaker layout.