In this lesson, I fit the curved dorsal fin into a curved slot in the back of the fish.
This is quite a long video I want to show you how carefully I work: In the end, you are looking for a tight fit, with the glue line of the joint hardly visible. You can only do this by working carefully; it's so easy to be impatient, or take off wood without real consideration and, suddenly, you have a loose fit.
So, softly softly...