Grass may appear to be deceptively simple, but in order to make your 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. Luckily, this doesn't mean that you need to paint every single blade of grass. 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.
With a palette knife using yellow and blue, mix a shade of green on your palette. Do not fully blend the green to a single flat shade, but allow some areas to be more yellow than others.
Quickly apply a wash of green to the canvas where the grass should be. You will need to move fast, because acrylic dries quickly. To make a wash, dip your brush in water and then add the water to the paint on your palette.
Mix more green and add a thicker layer to the green wash already on the canvas. Use a flat brush to apply the paint in upward strokes. 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 will 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.
Work quickly. Before the paint you've mixed has dried, dab a detail brush in the darker shade of green. Apply it to the canvas in clumps of thin strokes. 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 colour 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. This may seem obvious, but grass that is closer to the foreground will be larger, and grass that is farther away will appear shorter. 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.