Pick tunas when they are very ripe, almost ready to fall off the plant. Peeling is optional, because the tiny thorns will be filtered out in the final step. Mash thoroughly with wooden mallet and freeze, which will help break down remaining fibrous material. Thaw mash and push through a colander, then strain again through fine mesh. Juice from the first pressing will be very concentrated; it’s good for some recipes, don’t drink without diluting! You can get a second press from the same mash by adding water and putting it through the strainers again. Juice and concentrate can be frozen or used right away.