Use only fresh green leaves known to be edible, such as alfalfa, Swiss chard, lambsquarters, blackeye peas, wheat, mustard, kale, or collards. While many other plants make good concentrate, it is safer for beginners to stick with commonly eaten leaf crops. Wash in clean water to remove dust and dirt and cut into pieces 2 - 3" long.
This can be done with a manual meat grinder or flour grinder, a wheat grass juicer, or a household blender. Fruit and vegetable juicers usually clog up quickly from the large amount of fiber in leaves. I prefer using a blender on the highest speed 1/3 full of water. This step ruptures the cell walls of the leaves liberating protein and other nutrients.
Pour the pulped leaves into a sheer nylon or polyester cloth of the type used for curtains. Squeeze out as much juice as possible. You should not be able to squeeze any juice out of a handful of this pulp when you are done.
Stir very gently to prevent burning and remove from heat as soon as the leaf juice boils. A green curd should float to the top.
When this wet curd has cooled, squeeze the "whey" out of the curd. It should be dry enough to crumble. You may want to make a very simple press with a wooden 2" x 4" x 8' lever to apply more pressure than you can with just your hands. This can be used for pressing the juice from the pulped leaves as well.
10 lbs. of leaves should give you roughly 1/2 lb. leaf concentrate; 4 1/2 lbs. of fiber for mulch, compost, rabbit or goat feed; and 5 lbs. of "whey" for watering plants. If not used right away, leaf concentrate can be dried at about 120° F, ground to a fine powder, and stored for later use in airtight plastic bags away from any light. Good Luck!