How to apply silicone sealant
Silicone can be quite a tricky sealant to apply evenly and neatly. Firstly cut the end of the nozzle off at roughly a 35 degree angle (cut the nozzle to the desired size depending on the application). Load the silicone tube into the silicone gun and, keeping the flat side of cut nozzle against the area, gently squeeze the trigger until silicone squirts out. Use an old cloth or toilet paper to practice using the silicone until you feel comfortable and get a feel for what your doing, then starting at one end of the bath or worktop work your way across in one constant squeeze of the trigger (good luck!).
...... and if sealing a bath?
Fill your bath 3/4 full with water before you start applying the silicone as the weight of the water can cause the bath to move. If you seal the bath with water in it, when you come to use the bath later on then the silicone will stretch enough and it wont lose its seal. Make sure though that you leave the water in the bath until the silicone has dried/cured (check the manufacturers instructions).
I recommend using an acrylic based bathroom sealant for the inexperienced user as it is water based and can easily be wiped up with water and unlike silicone the more you play around with it the better it can look! However I wouldn't recommended use this for shower areas that are constantly getting wet.