This is a quick pickling recipe, therefore it needs to be stored in the refrigerator. I recommend using the berries within a couple of weeks.
I checked out numerous recipes on the internet and slightly adapted this one from Simple Bites, where it was adapted from Marisa McClellan's book, Preserving By the Pint. Recipes do get around and change a bit as they pass from person to person! (I amended the recipe in 2019 to make a greater quantity of brine.)
In a small pot, combine the vinegar, sugar, salt and peppercorns. Bring to a boil and stir until the sugar and salt are dissolved. Let the brine cool for a few minutes, then pour over the berries. The brine should completely or almost completely cover the berries.
Allow to cool uncovered, then cover. Shake the jar gently to redistribute the berries if they're not almost covered by the brine. Remove excess berries if necessary. Refrigerate. Allow the pickling at least 24 hours to take effect before using the berries. Store up to two weeks in the refrigerator.