how to prepare fresh sardines for bucatini con sarde
If you bought fresh sardines before to clean them and remove the backbone you should keep them in a bowl of water to get soft for an hour at least.

After you make the cut on the neck or simply remove the head with your hands.
Now there is the hole created by the head removal.
Enter your finger and open the sardina.
If you prefer to use a little knife cut from inside to outside the thinner part and open (thinner par is without the backbone). 
Remove the tail if you are using them for the pasta, otherwise don't cut it (vertical nor horizontal) if you want to fill them (for "sarde a beccafico").
Now your sardina should be open and still with backbone and little spine.
Since it became soft in the water should be easy to remove the backbone and little spines all together picking the backbone from the head part and separating it from the rest of the fish gently.

If you use stored sardines clean under fresh water gently. Remove Silver skin outside part and interior part. Remove the white long chain of backbone and eventually spines.

If you have any doubt about other passages or ingredients let me know.

Buon appetito.