Check out student examples here.
Click and drag out your canvas so it's about 1000 x 800 (make sure you see all of the canvas at 100%).
Zoom in to 400% using the slider at the bottom right of the window.
Select the Rectangle toolbutton and set the outline to the thinnest one.
Click on Pencil and draw a profile (side view) of a horse's head on one side (or bird or other animal can also work). Draw in a different color than your rectangle so you can see what's new and what's from the rectangle!
Be sure that the head touches the rectangle at both the top and bottom of one side. Has to be lower than the top line of the rectangle, and not go below the bottom line.
Right-click with the Pencil to get rid of the top & bottom parts of the left side of the rectangle (for both sides).
Then right-click with the Paint Bucket to erase those lines.
Make sure it'll work! If any of your new/drawn lines are below/above the original sides of the rectangle, start over or fix with the Pencil, carefully! In my example, my new/drawn line ends at the same height as the rectangle, so I'm OK.
When drawing on the third side of the square, draw your shape so that it fits inside the existing modified shape. Failure to do this will cause your shapes to overlap and then it will not tessellate. Optional: Add a little red dot with the Pencil to the top left corner of the rectangle so you can move it into place easier.
Select the entire shape and copy, paste, move it down into place. If there's any overlap, undo and draw the side over again.
Erase the black lines from the original square (right-click with Paint Bucket).
Use the Eraser to erase lines to get just one shape.
Fill shape with Paint Bucket. If it fills the entire canvas, undo and zoom in, find the gap, using the Pencil to close it.
To change the color of the border, say back to black to make an outline that stands out more:
Can also use the Paint Bucket to paint on lines, and use the Pencil on any parts missed.
Add details – outline first with black so have closed shapes, then fill with colors.
You want different versions of the same animal – different colors.