Make sure the fax machine is plugged in, powered on and connected to a working phone jack. This phone jack can either be your regular phone line or a dedicated fax line. The important thing is that the sender has the right number.
Make sure that the fax machine has enough ink in its toner cartridge. Toner cartridges usually have some sort of indicator when toner is low. Most modern fax machines will also alert you when toner is low.
Make sure that there's enough printer paper loaded in the fax machine's paper tray. Fan the paper (run your thumb along the bottom, separating the individual pages) to avoid paper jams in the machine.
If there's a phone on the fax machine, the phone will ring. Don't pick it up.
Wait for the "handshake" tones indicating that the fax machine is talking with the sender's machine.
The fax machine will automatically begin to print each page of the fax.
Check the coversheet to make sure you received as many pages as were sent.
If it's an important document, it's office etiquette to call or e-mail the sender to confirm that you received the fax.