Step 3: Cut the coffee filter
Now you must cut a circle of coffee filter that is small enough to fit inside of the cap, but big enough to rest on the lip that you left earlier... You left a lip, right?
The closer you cut the filter to the actual size of the inside of the cap, the better. When you place the filter inside of the cap and then screw the cap down onto the bottle, the filter gets compressed between the two. This keeps the filter in place, and only gives the liquid one path to exit the bottle.
To get the closest size filter, take the cap off the bottle and put it upside down on a piece of filter. Trace with a pen or pencil and then cut out. Make sure to cut on the inside of the line as so no ink or graphite makes its way into the moonshine.
Next, take the circle that you just cut out and see if it fits into the cap. If it does... congratulations. If it's too big... trim it. If it is too small.. start over.
I cut two sheets at a time, and place both in the cap. Like a double filter.