Sardine's hand opening

Scrape off the scales from the sardine with the back of a knife and cut off the head at an angle

Use a knife to remove some of the hard part of the belly (belly bone)

Insert your fingers through the cut in the belly and scrape out the innards

Wash the blood thoroughly with water

Hold the fish in your left hand and place the thumb of your right hand on the backbone. Slide your thumb along the backbone, opening it towards the tail. (If your thumb is not deep enough, the flesh will come off when you remove the bone in the next step. The key is to press firmly against the bone so that you can feel the presence of your right thumb on your left hand. However, be careful not to use too much force, as the flesh will tear.)

Once the fish is open, remove the tail bone and gently pull the backbone towards you

Once the backbone is removed, scrape off the remaining belly bones

If it looks like this, it's finished

[Reference]
You can also open the fish using only your hands, without using a knife. Insert your thumb into the base of the fish's neck and open the belly, then scrape out the innards and wash them. Then, open it along the backbone as described above. As you peel the backbone away from the tail, the fish's head will automatically come off attached to the bone.