Rockwool is a spongy, soil-less growing medium that is most commonly used in hydroponic gardening. However, you can also start seeds intended for transplant to soil in rockwool cubes. Obtain rockwool from a hydroponics store or online. Since you will not be planting in soil, you need to ensure that your seeds receive adequate nutrition in addition to water. If you transplant your seedlings from their rockwool cradle to soil, make sure to snip the rockwool away from the roots first.
Consider your tap water and whether your locality fluoridates it or not. Consider what seeds you are trying to grow in the rockwool. Use bottled water if your tap water has fluoride and the seeds you are planting are known to be sensitive to it.
Mix water and nutrient solution together in the amount recommended (on the back of the nutrient solution package) for the capacity of your tray.
Test the pH level of the solution and adjust so that it is around 7.0 for most plants. Adjust lower for plants that require more acidic pH levels, or higher for plants that need more basic pH levels. Dilute with water or concentrate with more nutrient solution to adjust. Buy pH adjustment chemicals made just for this purpose at your hydroponics store if you prefer.
Dip the rockwool squares in the solution, soak it up, and wring them out; rockwool should be moist but not dripping. Insert a couple of seeds into each slit in each rockwool cube.
Arrange the cubes in the tray or move to your hydroponic system setup. Fill a mister bottle with the mixed nutrient/water solution and keep near the tray. Cover the tray with plastic wrap and store in a place with filtered sunlight, such as through blinds or curtains.
Mist the rockwool cubes every few hours to keep them moist, but do not saturate to the point of dripping. Remove plastic wrap once seeds have germinated. Continue misting rockwool with nutrient solution and water.