Pour apple cider vinegar and drop of dish soap into glass or jar.
With the paper, make a cone with a 3/4-1 hole on the bottom. The cone should sit on the glass rim without the tip touching the vinegar. You may have to squeeze-shape the cone a bit to prevent gaps between the paper and rim.
Every day or two, switch out the vinegar. (You'll see when it stops being as potent. The flies/gnats will stop going in.).