What the wall is made of
A background color for the mortar, the bricks, shading, lighting and a few details. The palette is small: a main tone, two shades, two highlights and the background color. The background comes first, so here the board is already the mortar color.
Step 1: Lay out the bricks

Each brick is 9 pixels wide and 4 tall. Leave one pixel of mortar between bricks. For the next row, shift the bricks so the left side of each lower brick sits right under the middle of the brick above. If you're not sure, count four pixels from the edge.
In Aseprite I draw one brick with its bevels, then copy and paste it across the row, and copy whole rows down. Much faster than drawing each brick.
Step 2: Shading

The light comes from the top left. So the bottom and the right side of every brick are in shade. Then take the darkest tone and put it in the corners where the mortar lines meet. That small touch gives the wall depth.
Step 3: Lighting

Now the opposite sides: the top and the left edge of each brick get the light tone. Shade on one side, light on the other, and the flat rectangles turn into bevelled bricks.
Step 4: Details

The details are just one more layer of light: sprinkle a few pixels of the lightest color on the top bevels. It's good practice to keep shading and highlights on their own layers, so you can tweak them later.
Your turn: sand bricks

Same wall, new palette. That's the point of this challenge: reuse a sprite you already have, change the colors, tweak a few details, and you get a completely new one. Want more? Add some cracks like on the rocks, or push the highlights and shading further.