I have demonstrated the technique on a small sink but the same principals apply to sealing a bath with silicon or inside a window frame with decorators caulk.
If you are sealing a bath, before you apply the silicon fill the bath with water. The water in a standard bath will add about 100kg plus the occupants weight. This means there is a good chance the bath edge might lower slightly. Sealing when full ensures the cured sealant doesn’t pull away from the bath when it is used, maintaining a good water tight seal.
You cannot paint over silicon sealant so if you are filling or sealing around the inside of a door or window frame use Decorators Caulk, which can be painted when dry.
With practice you can apply very neat sealant beads without the use of the smoothing tool. I have demonstrated with one as it makes good results obtainable by anyone.
Finally wear old clothes, when you get Silicon or Caulk on your fingers, you will be tempted to wipe it on your tee shirt !!!