The gist of the job is to head to the place, find the ledger , and make the changes without anyone seeing you. Naturally, getting caught or killing anyone automatically fails this mission. Do your best to be SNEAKY.
An excellent tip would be to KNOW where the ledger is (its not a crime to FIND it) and only change it when you are HIDDEN . For smaller, locked up locations you could always wait until night when people are asleep as well and do a little breaking and entering.
Once you are done, return to Delvin to let him know. You will be given your cut (400 Gold) and the job is complete.