This can be done with a utility knife. Any contaminants like oil and grease could prevent the silicone from bonding so clean the area thoroughly with alcohol.
Pop the silicone seal. Most tubes have a thin barrier inside the tube to prevent the caulk from curing. Many silicone guns have a tool to do this built on them. However, if yours does not, then a long nail or something similar will work. Load the tube of silicone into the gun.
The tip should be slightly above the surface, nearly touching. As you begin to press the trigger, watch the flow of caulk. With one steady motion, move the caulk gun straight along seam, creating a uniform bead. Before flow stops, quickly release the trigger and begin to press again as you continue to create a uniform bead the entire length of seam. Do not stop until you reach the corner.
