Cut the nose of the screw on nozzle at an angle and make the hole big enough to fit over both sides of the joint. Do this with it screwed off of the tube of silicone. Mask the wall with quality tape, don't worry about the glass as it'll just rub off when dry.
Screw the nozzle on when you're happy with the size and angle. Do a dry run over the joint with it all in the skelly gun to get used to the run you're going to make.
Breathe, relax and don't clench.
Pull a business like face, so the silicone doesn't know that you don't know what you're doing...and...
Fully fill the beginning of the joint starting at its lowest point making sure the silicone is squeezed against both edges of the joint with no air gaps, this will make it stick in place, squeeze enough silicone in the joint to fill it completely plus a bit more whilst moving the nozzle of the gun slowly along the joint with the nozzle pointed slightly back towards the start of the joint
Do the whole run without going over the old stuff.
Once you've filled the whole joint, get a spray gun with soapy water in it and spray the whole bead of silicone liberally. Take a wet spoon and run it along the joint, scooping up all the excess silicone and dump it in a bucket with water in it or plastic bag. Silicone won't stick to a wet surface, so it's easy to get it off the wet spoon and into the bucket.
Once you've scooped it off and the joint looks beautiful and smooth, pull off the masking tape, drink gin and relax.