Trim the greens and roots off the radishes, then rinse them clean. Chop radishes into quarters.
Prepare a brine by mixing 1 Tbsp sea salt or Himalayan salt into 2 cups (½ litre) filtered water. Stir to dissolve the salt.
Fill your clean fermenting vessel with the radishes, sprinkling mustard seeds over if you like. Rest your glass weight on top of the radishes and then pour the brine over so it covers the radishes and weight by 1-2cms, leaving a couple of cms of airspace between the brine and the lid. If you don't have enough brine, you can make some more using the above ratio of salt to water.
Clamp the lid on, fill the airlock to the fill line (around half-way) with filtered or cooled boiled water, and insert the airlock into the gromit-lined hole in the lid so it pokes out a bit underneath the lid. Leave your radishes to ferment at room temperature, out of sunlight, for around 7 days.
When they are ready, open the lid, eat a couple of radishes, cover the jar, and then store it in the fridge. It will keep for much longer than you need it to - ours never last longer than two weeks!
These instructions have been written for the 850mL fermenting kit I sell in the Mrs Goodness shop. The airlock means that fermentation gasses can escape the jar without you having to release or remove the lid. This just reduces the likelihood of contamination, but it is not an essential as long as your utensils and workspace are clean and your veggies are completely submerged under the brine.