1. Prepare fish fillet. Cut into slices. Season with salt and pepper.
2. Coat fish with a thin layer of flour.
3. Prepare batter: 1/2 cup flour, 1/4 cup starch, 1/2 tsp baking powder, 1/4 tsp salt, 1/4 tsp baking soda. Add in water (about 150 ml) while stirring it. Add in 1/2 tsp vinegar at last.
4. Dip fish in batter first, then flour again.
5. Deep fry till golden brown color.

https://flic.kr/p/2h6HMvo

If you want it to be crispier, you can fry it twice.