Wipe off the surface where you are applying the caulk with a damp rag, then dry it off.
Apply strips of blue painter's tape along both edges of the seam where you're applying the caulk. Keep the strips 3/8 inch apart.
Load the caulking gun. Drop a tube of caulk into the gun, then repeatedly squeeze the trigger until the plunger reaches the back of the tube. Cut the bottom edge of the nozzle off at a 45-degree angle with a utility knife. Break the seal inside the nozzle with a screwdriver or nail.
Place the tip of the nozzle along the seam at one end. Squeeze the trigger with steady pressure and slowly glide it along the seam at a steady pace. When you completely depress the trigger, lift the caulking gun off of the seam and release the trigger. Place the nozzle back on the seam where you stopped, and continue applying the caulk.
Wipe a damp paper towel over the seam, pressing your finger into the seam and wiping the caulk in a smooth, steady motion.
Remove the painter's tape and smooth out any bumps that were created by removing the tape. Allow the caulk to set for at least 24 hours before using the shower.