Remove any wood.
Remove the grate that holds the wood.
Shovel out ash and small bits of wood.
Wipe down the walls with a stiff brush.
Vacuum up all the remaining debris from the bottom of the insert and walls.
Clean glass doors if you have them so that they will shine.
Remove the insert. Depending on the setup, you might have to slide the insert out.
If it has a liner that extends all the way to the top of the chimney, you do not need to remove the insert for cleaning purposes. If there's no liner, you will need the insert out of the way to clean the chimney.
It might take more than one person or a professional chimney sweep to pull it out. Place the insert on old sheets or tarps to keep your home clean.
Contact a chimney sweep, if you haven't already, to clean the chimney portion of your fireplace. This only needs to be done every few years.
You can clean out the insert as often as you want.
Put the insert back in the fireplace when the chimney sweep is done, and replace the grate and wood.