I have created a separate excel file with columns for existing and desired field names.
But I am unsure how to implement the Mapping Table. I have tried inserting the code provided in the link you attached but I receive an error message when I attempt to load the data. This reads: Field 'Desired' not found.
I am now unsure how best to proceed, please can you help me.
In general it's not recommended to make any transformations within a crosstable-load. It's a heavy and complex process and to add further adjustments leads rather to problems or performance issues. Therefore the common solution is to make this afterwards in following loads/statements.
Beside them in your case you might get nearer to your requirement by following this pattern:
crosstable(Expenditure.Date, Expenditure.Value, 1) load * from ExpenditureSource;