Grass appears deceptively simple to depict, but in order to make a painting more realistic, you'll need to do more than lay down a flat green blob on your canvas. The realism is in the details. This doesn't mean that you need to paint every single blade of grass, though. The most important thing to remember when painting grass is to be patient and thorough.
Prepare your materials. If you are working from a live subject out of doors, do not bring anything that can easily be blown away. With a pencil, sketch the image of your painting onto the canvas.
Mix a shade of green on your palette using yellow and blue paints and a palette knife. Do not fully blend the green to a single flat shade, but allow some areas to be more yellow than others.
Dip your brush in water, add the water to the paint on your palette and quickly apply a wash of green to the canvas where the grass should be. Move fast because acrylic dries quickly.
Mix more green and add a thicker layer to the green wash already on the canvas. Apply the paint in upward strokes using a flat brush. Don't worry if some sections are lighter or darker than others -- variations in the tones make the grass look more natural and less flat.
Mix a slightly lighter shade of green and a slightly darker shade of green. You need less of this than the green you already mixed. Remember, more yellow will make the green lighter and more blue will make the green darker.
Dab a detail brush in the darker shade of green and apply it to the canvas in clumps of thin strokes. Do this quickly, before the paint you've mixed has dried. Scatter the clumps of grass all over the layer of green you've already laid down.
Add the lighter shade of green to each clump of darker green strokes. Make the lighter strokes longer and more prominent.
When mixing green, mix blue into yellow, not the other way around. This will prevent the color from getting too dark too fast.
For longer blades of grass, use a detail brush with longer bristles. For shorter blades of grass, use a brush with shorter bristles. The shorter bristles will give you better control, and the longer bristles will hold more paint.
Paint larger grass closer to the foreground and shorter grass that is further away. This will give your painting a greater feeling of depth.
Grass in a field will likely be much more yellow-gold than grass on a lawn. Grass in a field will also be taller and in comparative disarray.