What is the correct technique for calking two adjoining surfaces in a shower?
Yes, YouTube is great at showing the easiest of circumstances. What has worked for me is cutting the caulk nozzle straight across instead of at the 45 degree angle they suggest. Cut at the tip for the smallest opening and then press the caulk tube firmly against the tile seam at 90 degrees and start caulking. Move the tube slowly along the seam. If some caulk squirts out don't worry about it, wait until you're done with that section and then clean/smooth the seam with your finger, working more caulk into the seam. It will get easier once you get to the larger gaps. If you can weight the shower pan down you might be able to widen the gap before caulking.