Lay jar on its side and pack radishes tightly into the jar. Make sure the jar is well cleaned.
Cover with brine, leaving at least 1 inch of headspace.
Put an airtight lid on the jar.
Place the jar on the counter at room temperature. You'll need to burp the jar once, or twice a day to release pressure. To do this, slightly loosen the lid to let off gas and re-tighten.
Ferment for 5 days - 2 weeks, or until they are the flavor and texture you desire.
Radishes will ferment a bit faster in the summer than in the winter. A thinner size slice will also ferment faster. I generally julienne them and ferment for around 5-7 days.
Store in the refrigerator for a month, or two.
The amount of salt is personal preference within a given range. Start with 1 1/2 tsp. and increase to taste, if desired. If too little salt is used, the vegetables will not be crisp, it will also not discourage the growth of mold. However, if too much salt is used, it can slow and even prevent fermentation from occurring. Salt within the given range of 1 - 2 1/4 tsp. per cup of non-chlorinated water will keep your brine in a healthy range.
You may reduce the amount of salt further, by using 1 Tbsp. whey, or juice from another ferment, per 1 cup of non-chlorinated water. Whey tends to be a little more predictable. I would not recommend reducing salt below 3/4 tsp. per cup of non-chlorinated water.