Wash 2 quart-size glass jars and lids with hot soapy water and rinse well. Rinse the cucumbers, scrub gently, and trim a thin slice off the blossom end of each.
Make the brine by stirring the pickling salt into the water until fully dissolved.
Divide the garlic, fresh dill, mustard seeds, and black peppercorns between the jars. Pack the cucumbers snugly into the jars, standing them upright.
Pour the brine over the cucumbers to cover by at least 0.5 inch. Leave about 0.5 inch headspace. Add a fermentation weight or a small clean zip-top bag filled with some brine to keep cucumbers fully submerged. Loosely close the lids or use airlock lids.
Ferment at cool room temperature (68–72°F/20–22°C), out of direct light. Check daily to ensure everything stays submerged; skim any harmless surface yeast.
Begin tasting on day 4. Pickles are ready when bubbling slows, the brine turns cloudy, and the cucumbers are pleasantly sour and olive-green through the center, usually 5–10 days depending on temperature and size.
When they taste right, remove the weights, tighten the lids, and refrigerate. Keep cucumbers submerged in brine. Chill at least 24 hours before serving; flavor deepens over 3–7 days. Consume within 1 month for best texture.