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.