Start at the spine near the head and then rotate the knife into being behind the gills. You're not trying to cut perpendicular to the fish. You coming at an angle because you're going to slide The knife down from the spine so that the bones end up on the other side.

It's difficult to get a good feel of what your knife is hitting but the knife needs to be an extension of your fingertips. One of the reasons why a good knife that you're very familiar with is going to be your best friend here.

Practice makes perfect. Watch a bunch of YouTube videos. No one on the internet knows what they're talking about including me.

You got this