How can I make fine powder from leafy herbs?
Mill them.
When you dry them very well ( so when you think they are dry put them in oven for 20 minutes at around 40 Celsius). 

Then use mortar. This is it exact use in kitchen, to turn moist things into paste and dry things into powder. 
If you don't have one use poor man hand mill.
Put leaf on large cutting board and then use smaller one on top. Move it in circular motion with little to no pressure.