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Do not fill the mortar more than 1/4 full. If it's too full, then you won't be able to fully grind it. Use the pestle to thoroughly grind the blades until they begin to stick together and smear along the bottom of the mortar. Apply the pestle using a stirring motion, and press with enough force to crush the grass. This will take a few minutes, and more than a bit of effort, so be prepared. Equal parts water should be sufficient for this method. Mash the water into the crushed wheatgrass using the same forceful stirring motion as described above. Continue mixing until a fine paste forms. The water will help you sufficiently grind down the blades of grass. Twist the top of the cloth to prevent the paste from escaping, but do not tie it. This will allow you to extract the juice from the wheatgrass. Apply pressure to the cloth directly above the clump of wheatgrass paste, squeezing in a downward motion. A bright green liquid should drip out. Continue squeezing until no further liquid drains. Repeat the grinding process until the blades turn white, adding a little extra water each time to create a paste of the same consistency. Continue until your full 1/2 cup (113 grams) has been used. This process does take a while, (at least 10-15 minutes for 1/2 a cup), but it'll be worth it. It's much better than paying $300-$400 for a fancy wheatgrass juicer.

Summary:
Place enough chopped wheatgrass into the mortar to fill the bottom. Grind the blades. Add a little water. Empty the contents of the mortar into a clean muslin cloth. Press on the cloth to drain the wheatgrass juice out into a clean glass. Return the wheatgrass clump to the mortar. After your initial batch of wheatgrass turns white, add fresh chopped wheatgrass to the mortar and begin the grinding process again.