You have to remember that LBD is in Australia so he always cuts his dovetails upside down.  He's also still a little perturbed that his effort to change the name from dovetails to ducktails never got any traction.

In all seriousness, yes, you will have gaps on your first several attempts.  My preferred method of fixing gaps is super glue and saw dust.  Pack the saw dust into the gap and wick a little super glue into it.  Let it dry and sand it flush.

Two things that really helped me in my early efforts to master dovetails with hand tools:
  1. Use painters tape a'la Mike Pekovich instead of relying on my less-than-perfect eyes to be able to saw on the edge of a knifed line.
  2. Practice makes perfect.  I did "30 days of dovetails" (well I called it 30... it was more like 10/30 if I'm honest) where I took a scrap 3/4" board about 4" wide and cut it into a bunch of pieces about 12" long.  Every evening I would take two boards and layout and cut mating dovetails.  Then I'd cut an inch or two off that end of the boards and label them "Day 1."  Then the next day I'd use the rest of the boards and do it again.  Rinse and repeat.  There's no better teacher than experience.  It only takes about 20 minutes a day once you learn to lay them out efficiently and if you can take that 20 minutes for 30 consecutive days, you'll be pounding out pro-level dovetails with your eyes closed!