After opening an envelope, decide which category the mail should go. Is it a bill that needs to be paid? Put it in the "To Pay" file and mark the due date on the calendar and the outside of the envelope. Keep bills in their original envelopes until they are paid to keep loose items together.
Is it a bank statement that you need to reconcile or other document that you need to take some action on? If you cannot take care of it immediately, it goes in the "To Review/Act On" file. You should set a specific date and time each week to review the contents of that file to help manage the influx.
Use the calendar if you need a visual reminder of when you need to do something by.
Sometimes you'll receive items in the mail that you will need to hang on to for future reference, like tax statements, receipts, etc. These go in the "To File" category. This category should be checked weekly and items placed into a filing solution.