From 12 years of art classes, I learned to always begin by blocking in basic shapes of color. Don’t jump into detail or you will probably regret it later. My art teacher always told me to work the WHOLE picture and not just one little spot.
For the grass, it helps that the fabric is already green, but unless you add other colors to it, the grass isn’t going to look real at all. Here I use some yellow for the highlighted parts and brown for the shadows. My lamb is just a couple splotches of light blue paint. You can see that for the fields beyond the hill, I mixed a little yellow and white together and painted in the basic shape. In some patches, I didn’t put the paint on very heavy because I wanted the green to show through. I let that dry over night.
Note: I had never painted on cotton before, so I was surprised to find out that paint does not spread very well over cotton. It takes several coats and lots of patience before it gets easier to work with. I recommend putting cardboard underneath the article of clothing you are working on, because the acrylic paint, if layered on too thickly, will bleed through to the other side.