I have access to fresh anchovies, so I use them. You could however try this with sardines or pilchards or sprats or maybe even herring or smelt. I haven't attempted boquerones with these fish, however. Keep in mind that my ingredients are approximate: This is not an exacting recipe, and I typically do this by feel. You can also change up the seasonings if you want, too. Have fun with it!
1cupred wine vinegaror lemon juice, or sherry or white wine vinegar
1to 5 small hot chiles, sliced thin(optional)
Keep the cleaned anchovies whole for now. You can pull the two little fillets apart when you are ready to serve your boquerones. Lay down the salt in a wide, lidded, non-reactive container; I use a tupperware. Lay down a layer of anchovies, then sprinkle salt over them. Repeat until you have all the anchovies in the container. Finish with the rest of the salt. Jiggle the container a bit to let things settle, then cover it and put it in the fridge for 3 hours.
Remove the anchovies and rinse them quickly under cold water. Set them in another container like your first one. When they are all nicely rinsed, cover the fish with the vinegar or lemon juice. They need to be fully submerged, so use as much as you need. Cover and refrigerate overnight.
The next morning, remove the anchovies from the vinegar. Toss the vinegar. Now find a final resting place for your anchovies, some lidded container that you can keep in the fridge for a few weeks. Pour a little olive oil into it, then line the container with a layer of anchovies. Sprinkle a little of your seasonings over that layer and repeat until you are done. The anchovies need to be completely covered in olive oil or they will spoil. You can eat them after a few hours, but they are better after a few days or even weeks. Mine have lasted 6 weeks before getting a little janky.