Using Silicone Sealant
Apply masking tape 1/8 in (3 mm) from each side of the seam (Image 1). This will ensure that the sealant will have straight edges when finished. Cut the nozzle of the sealant tube at an angle, so that the diameter of the opening is slightly wider than the gap that needs sealing. Follow the instructions supplied to load the sealant tube into the dispenser.
Pressing evenly, squeeze sealant into place, moving slowly along the joint. Apply a little at a time, so you can smooth it as you work (Image 2).
Use paper tissues to remove any excess sealant from the nozzle. Smooth the sealant as you work by running a wet finger along the joint (Image 3).
Remove the masking tape to reveal a straight sealant band along the extent of the seam (Image 4).
If necessary, smooth over the sealant once more, pressing gently to avoid spreading it beyond the neat edges created by masking (Image 5).