There are two ways of preparing herring, depending on how you buy them.
If you buy herring in tin cans, the first thing you must do is de-brine them as they are way too salty. Place them in a large pot filled with plenty of cold water for two hours. Then get rid of the water.
If you buy fresh herring, on the other hand, place them in a brine in the fridge overnight, see ingredients above.
Remove the herring from the water, and let the water drip off. Cut the herring filéts in bite sizes. Mix the herring with all ingredients in a glass jar. Keep it in the fridge.