Fresh sardines are a bit of work to prepare. Small fish usually are, but don't let that deter you. They're worth the effort. And once you get it down, the process goes quickly. You don't even need a knife to fillet a sardine. Their flesh is so soft they can be filleted with your thumbnail. In fact, doing this will help you preserve the texture. By filleting sardines this way, you force the bones out of the flesh. When you cut sardines with a knife, you cut through the bones, leaving them behind in the fillets.
Start by gutting the fish. Place the fish on a clean napkin with the belly facing you. Use your thumbnail to separate flesh from bone where the anal fin meets the belly.
Work your thumb under the flesh while sliding your thumb toward the tail of the fish. At this point the flesh of the sardine should be separated from the bone on the first side.
Slide your finger on top of the backbone toward the head of the fish to separate the ribs from the fillet. At this point you will be able to see where the backbone terminates at the tail. Pinch the backbone of the sardine between your thumb and pointer finger and break it.
Gently lift up on the severed backbone, and it will pull free from the flesh, bringing many of the smaller bones with it. When you get to the head of the sardine, you can pinch the backbone again to sever it from the head and lift it free. Whether you leave the head on the fish is up to you.