Combine fresh ground coffee and filtered water (3:1 ratio by volume, leaving 3/4-1 inch head space) in a Ball jar or other container with lid. Seal lid and shake for 10 seconds.
Loosen lid and let rest for 20 minutes.
Tighten lid, shake, then let rest for 36-48 hours in the refrigerator.
Once the coffee is full steeped, place coffee filter in cone with mesh strainer on top. Gently pour the cold brew into the starainer/filter combo, being careful not to dislodge the grounds from the bottom of the steeping jar.
Once all the liquid has been poured off, let the jar rest for a few minutes. As additional cold brew starts to separate from the grounds, pour the liquid off once more. Finally, give the spent grounds a light rinse with about an 1/8-1/4 cup of filtered water to flush out any remaining cold brew from the grounds, then pour the liquid over the filter one more time.
Put a lid on the jar and store in the fridge for up to one month. Serve black over ice or mixed with your choice of plant milk.