Small, oily fish, such as the sardine shown here, have such soft bones that they can be boned with your fingers rather than a knife.
With your fingertips, break off the head of the fish behind the gills. Discard the head and gills. Insert your forefinger into the head end of the fish and run it down the belly so that it slits open. Working from head to tail, pull out the innards and discard them.
Open the fish out and, working from head to tail again, pull out the backbone. Release the backbone at the tail end by snapping it off with your fingers. Rinse the fish thoroughly and pat dry with paper towels. The fish is now ready for cooking.