This stage is a great exercise, almost a woodcarving 'boot camp'!
I spend quite some time showing you how to lay out the lines, the ridges between 'flutes'. Take your time on this; it defines the carving to follow.
Before you tackle the fish itself, do practise 'fluting' on waste wood.
Sharps tools! Did I need to say that? I'll say it again anyway: Sharp tools!
Make shallow cuts to begin with; use your tools to find the lie of the wood and react accordingly
'React' means stopping if you find yourself tearing up the grain and reversing the direction of cut so as to clean up the torn side.