Find a design online and print it off so its the right size for whatever application you're using it for. Black and white designs with hard lines work the best, also simple is better or you'll be spending the whole day cutting it out with an Xacto knife.
Find some stiff, thin cardboard and tape your paper design onto it. Cut out the stencil with an Xacto knife.
Get a cookie and place it on a flat surface. Align the cardboard stencil on top of it. Get a sifter or a sieve and spoon in some dark cocoa powder. Sprinkle the cocoa onto the stencil so the pattern appears on the cookie. Carefully remove the cardboard so you dont smudge the design or spill cocoa powder off the cardboard onto the cookie. Brush the excess cocoa powder, and repeat.
Dont try and stencil cookies that are already arranged on a plate. When you put down the stencil on one cookie, you'll end up setting it down on cookies with the cocoa powder already stenciled on, and it will get smudged and messed up. Stencil one cookie at a time and then transfer the cookie to the plate afterwards.
Get creative by using two tones of cocoa powder, white protein powder, or powdered sugar. Also, try using two stencils on top of each other for a three-color image.