I grind fresh for my Aeropress, but use pre-ground (or grind-at-the-store) for cold brew and try to make it that day. You can do it in a large mason jar with several paper filters, but I use a Toddy system I got as a gift.
Basically it's 12 OZ ground coffee + 7 cups of water in the bucket. I let that sit on the counter for 12-24 hours, then run it through a very fine filter (~1 cm thick piece of felt). This produces a concentrate that I store in the fridge. Dilute 1:1 or 1:2 with milk and/or water before drinking. It's good heated or over ice.
Basically it's 12 OZ ground coffee + 7 cups of water in the bucket. I let that sit on the counter for 12-24 hours, then run it through a very fine filter (~1 cm thick piece of felt). This produces a concentrate that I store in the fridge. Dilute 1:1 or 1:2 with milk and/or water before drinking. It's good heated or over ice.