Fire pan use is pretty straightforward.
A lot of people do not realize until they've used one, but the reason for building a fire in a fire pan is to contain the ashes so they can be packed out. Since you're packing out ashes, you'll really want to consider whether you need that last log at the end of the night. Coals must completely burn out before you can pack them up in the morning.
Assemble your fire pan before lighting a fire in it. Most firepans have two legs that fit through slots welded into the firepan.
Always set up on a fire blanket so stray coals don't scorch the beach. Once assembled stable and upright, build a fire inside like you normally would in a fire ring.
Always stir ashes before throwing new wood on. Hot coals get covered up in ash and don't completely burn, which can become a problem when you're packing the ashes into your ammo can in the morning. Use only what you need when using water to extinguish the last few remaining coals at the end of the night. The longer you keep the small coals burning, the more likely you'll be scooping them as ash instead of half-burnt wood chunks in the morning.
Lastly, never pack hot coals that came out of a firepan in a metal ammo can, make sure they are completely cold to the touch before packing away. Failure to do this means you could melt a hole in your boat or gear bags that are in contact with the metal can.