Use a paper filter as a pattern. Place one edge along a folded piece of pre-washed natural 100% cotton muslin (avoid bleached or dyed materials). Add an extra 1" along the top edge if you want a finished edge.
You will wind up with something like this when it's unfolded.
Sew a hem along the top edge of the open fabric (if desired). It works fine without one, but looks more finished this way.
Fold your filter in half again on the fold line. Put the right sides together if you've done a hem. If no hem, it doesn't matter which side is out. Sew along the two open edge seams.
Now you have a new 100% reusable coffee filter! This coffee filter will prevent grounds (and cloudiness) in your coffee, it will taste great, and is easy to rinse out. It stays put in the coffee maker and you will have no waste aftercomposting the grounds. If you'd like an even more finished look, you could add seam binding to the top.
Use the finished side IN so that the coffee grounds don't get stuck in the seam. Here's one after a few week's use (notice the un-finished edge still is working fine, just not as pretty!). Just rinse out, and lay out to dry. It dries very quickly and is ready to use again the next day, however having two is probably better so you can rotate them. Since it's cotton, you can also machine wash and dry it if you want to.