Firstly you need to remove some bones from the fish. They run down the centre of the fillet, and are rarely found in the narrowest third of the fish.
Run your finger from the wide end down the centre and you'll feel them. Grab each one and draw out. It's not difficult but it is a little time consuming.
Once all the bones have gone, remove the skin.
Pop the meat in a bowl, and ideally mush it with your hands to feel for any bones you may missed.
Add the creme fraiche and horseradish, and mix well.