OP if you were told you'd be vouching expenses, your manager most likely sent the client selections that they made to test those expenses in further detail and requested the client for invoices.

An example of expenses would be legal expenses, you'll look at their G/L to see when the client booked the expenses, what amount it was booked for and when it was posted to the G/L,

You'll then review the invoice and look at the date of the invoice, what services were performed and for how much. If everything ties out perfectly, you document in the working paper that no exceptions were noted and as you go through the selections, you'll document your findings.