You will need something to hold your cocoa powder. I use a new white "footie". Fill it with cocoa and secure with a rubber band. I keep mine in a plastic zip lock bag, so it is always on hand.

Lay your pattern(s) on a flat piece of styrofoam and cover with waxed paper. Take a hatpin and poke holes on the designs seen through the waxed paper. It's kind of like making a stencil on waxed paper.

Then after the icing has crusted on the cake, I take the waxed paper pattern(s) and place on cake. Secure the waxed paper pattern with toothpicks. Then I take the "cocoa puff" footie and pat or gently rub over the waxed paper until all the holes are covered and the pattern is visible. Remember to cover all areas of cake where you DON'T want the cocoa. When done, carefully remove the waxed paper so as not to get the stray cocoa on the fosting. Lifting straight up works best. There are several other ways to transfer patterns; I just happen to like this one.

Now you are ready to pipe or "paint" icing on the cake, or use Wilton colorflow or star tips or whatever you want. Brush embroidery looks good on flowers and leaves.