Remove the scales. I have a mackerel in the picture so I don't really need to, but I listed the step here.

Make an incision into the gills to make it easier to remove them.

Slit open the belly.

Remove the gills and the guts.

Clean out the insides.

Make an incision with your knife along the line connecting the caudal fin and the ventral fin.

Make an incision on the other side as you did in Step 6.

Cut off the head.

Cut into the flesh from the belly side and slice.

Pierce through to the back side until right before the tail fin.

Cut into the flesh from the back side and slice.

Insert the knife where you pierced through in Step 10, and slice toward the head to remove the flesh.

Slice through to the tail.

You have two pieces now.

Cut into the flesh from the back side and slice.

Pierce through to the belly side near the tail fin.

Cut into the flesh from the belly side and slice.

Insert the knife where you pierced through in Step 16, then slice toward the head and remove the flesh.

Slice through to the tail.

You now have three pieces.