Now, for the Fun Part!
To open the caulk, most professionals use a utility knife.
Hold the caulk with the tip facing away from you. Place the blade of the knife less than .5" away from the end of the tip, angling it away from you as well. With one strong motion, push down and out -- the tip of the caulk should go flying off, leaving behind a perfectly angled end.
This is the way the Real Deal guys do it, and may take some practise. You can manage the same thing using tough scissors or wire cutters. Just keep in mind that the tip should be cut off very close to the end, and at an angle.
Next, the gun.
The end of the gun has a metal hook on it: this hook needs to be pulled all the way out, and as you work, it will move forward. Push down on the metal lever while pulling the hook end all the way.
There is a little metal piece on a hinge on the handle. The sole purpose of this part is to finish opening the caulk. Pull it out, and stick it into the hole in the tip you've just cut. You'll feel the resistance; break it.
Now push the caulk into the gun, tip-side first.
There are two parts to the handle. Squeezing these together is how you push the caulk forward. Unless the pressure is released the caulk will keep oozing out messily; to release the pressure, just push the metal lever you used to pull the hook out.
(At this point, you'll want to have some paper towels or cloths nearby.)
Position the tip against the crack you're filling. Squeeze the handle gently, and begin moving along the edge as soon as caulk begins to come out.
Remember: Caulk is not icing. Use very little.
After it's applied around the edges, put the gun aside (remember to release the pressure).
To finish it off, I personally don't mind getting a little messy; some might want gloves. I wet a finger and press it hard against the corner, then move along the crack, wiping the caulk off the finger and rewetting as I go.
What you want is a concave bead inside the corner. *Less is more.*
And that's it!
Leave the area alone for however long the label says; overnight is a good start. Do NOT get it wet, or it'll make a big freaking mess.
Because I lived with the 4 other people, who hadn't asked me to take on this job, I had to work something out so they could shower. I rigged a plastic tent involving Saran wrap and (white) duct tape ... it was OK in the end, but it's technically naughty caulking behavior ... so you didn't read it here.