Get a fermenting vessel. (Read about my favorite fermenting jars here.)
Slice or dice your radishes. Layer them into your jar with some fresh dill and garlic, if using. Depending on the size of your jar, anywhere from 2 smallish sprigs to 5 large stems. Really jam it all into the jar, pressing down as you go.
Dissolve 1 tbsp of sea salt per 2 cups of water. Pour over your packed radishes. If you have it handy, you can also add 1 tsp of whey, fermented sauerkraut juice, any juice from a previous ferment, water kefir, or the "water" from a yogurt with live cultures. This however optional. It just speeds up the fermentation.
Once you filled your jar with brine, use a skewer to poke down the side of the jar to release air bubbles.
If you're using a Fido jar, no need to use a weight to keep things submerged.Once closed, it won't matter if some starts to float up from under the brine.
If you are using a different fermentation vessel, submerge the radishes under the brine with a fermentation jar.
Place your fermentation jar on a plate or in a bowl. Once fermentation gets going, there will likely be some leakage.
Now let them sit and ferment at room temperature for 7-14 days, depending on the temperature in your kitchen. If you used some fermented liquid to get it going, you'll see fermenting action much sooner!
The radishes will also change color if you're using the red and white variety, to quite a vibrant pink. I usually leave mine for about 7 days, but bear in mind I speed it up and my kitchen is pretty warm.