Step 4: measure aluminum powder
"Fluff" your aluminum powder by agitating it with a spoon or similar instrument. This serves the same purpose of the milling did with the perchlorate: The powder will not be clumped together, and it will mix better, making a more reliable explosive.
Again, using your scale, measure 1/3 of your batch size in aluminum powder.
Carefully pour the aluminum powder into the mixing bag with the perchlorate.