The quickest and least painful and stressful to the fish is to net it out onto a piece of paper towel folded over several times and placed on a solid surface, then fold the towel over the fish and give it a very hard wrap on the head (the whole fish with a small guppy). This instantly kills it. You don't see anything, and the fish does not suffer any more than it would being netted out for any other method which can be much worse for it, if we care abouot how it may perceive and feel things.