Wet your finger with some soapy water in a dish or basin and then smooth out the seam. The soap prevents the silicone sealant from sticking to your finger. You can also apply some soapy water with a plant spray. Remove any remaining traces of sealant with the soapy water.
What do you use to smooth silicone caulk?
To make it easier to spread silicone caulking: Apply a fine mist of denatured alcohol to the caulking bead using the spray bottle. Use your finger to smooth the bead of caulking with your finger before the alcohol evaporates.