Place the starter culture in a dish of cool filtered water. (Follow the directions on your specific package.)
Put the salt in a bit of warm water to dissolve.
Cut the radishes. I like to cut small ones in half and larger ones into fourths or sixths.
Put the radishes and starter culture in your clean mason jar.
Pour the salt water over the top. Add more filtered water if needed to make sure the radishes are completely covered, but leave an inch or so at the top to allow for expansion.
Place your fermentation weight on top of your radishes to keep them under the brine and then your fermenting lids on top.
Let the radishes sit in a dark place for at least 3 days and up to a week. (Test to find when it is fermented to your liking.)
Transfer to the fridge to slow the fermentation process. These will easily keep in the fridge for 6 months or more, but I highly doubt they will last that long! You can experiment with adding different herbs if you like.