The simplest and most important point is to use anhydrous ethanol.
Otherwise, any amount of water in the ethanol, even a little amount, will immediately cause the oxidation of your iron powder. Purchase small bottles of the anhydrous ethanol and immediately close the bottle after pouring out the amount of ethanol you need.
The funnel and filter paper you use should also be kept in a dry box before use to prevent them from collecting any moisture from the ambient environment.