In conjunction with the other guide on this subject, the following make for a perfect seal without the need for a steady hand :-
1 Use a mould resistant silicone sealant and do not be afraid of the more expensive brands, cheap sealants are often inferior.
2 Dip your finger in clear handwashing solution before smoothing out. This will prevent any sticking to your finger and ensure the surface is very smooth at the end
3 Use masking tape to define the line to be sealed. This will prevent any excess sealant ending up where it is not wanted. After removing the masking tape, run a finger along the sealant again. This will smooth out the ridges left by the tape and gives a second seal, further protecting against leaks.
4 When sealing onto tiles, ensure that the grout goes to the bottom, otherwise the water may find its way behind the sealant and leak into the room below.