How the ground is built
Look at the structure first. There's the top of the ground, which is lit. Under it there's a thin shadow, because the top sticks out a little. And then the body, with a bit of texture. The ground uses five colors: a mid-tone, one shade and three highlights. Grass, if you want it, gets its own greens.
Step 1: Outline

Really nothing to it. Start at the level where the ground should be, draw a 15 pixel line, go one pixel down, one pixel in on a diagonal, then straight down, and close the shape. Draw it on its own layer.
Step 2: Color

Fill the block with the brown mid-tone and recolor the outline the same.
Step 3: Shading

All you need is one line of the dark color right under the top edge. It shows that the top sticks out. Then sprinkle a few dark pixels into the body for texture.
Step 4: Highlights

The light comes from the top, so the top of the ground gets the highlights. Fill it with the darkest of the three highlight colors, add a line of the second one, and finish with a few pixels of the lightest. I leave three pixels, light the fourth, then do three more. Those little details give the ground a crisp look.
Add grass

Here's a small challenge. Use the greens to put grass on the top, with its own highlights and a bit of shading. Draw the grass on a separate layer. Then you can switch it on and off and get two ground styles from one sprite.
Build a scene

Copy the ground, paste it, and flip it horizontally (Shift+H in Aseprite) for the other side. Then move a few highlights so the two pieces don't look like copies. Once you know the basic structure, you can make any variation: a floating platform, a ledge, a block with the corners cut off. Add the cloudsand you have a little level.