Start by cutting strips of fabric in random widths. Ours were between 1.25 and 2.5 inches. This is a great way to use of all your scraps. Also cut an 8 inch (or which ever size you would like your block to be) square from white cotton. We used a thrifted cotton sheet.
You are ready.
Lay your first strip diagonally across the square, right side up.
Then choose your second strip and lay it right side down on top of the first strip. Make sure one of the sides matches up. (They will probably be different widths.)
Now stitch down the matched up side with a 1/4 inch seam. I have a 1/4 inch foot on my sewing machine, but for a block like this the 1/4 inch seam allowance isn't overly important.
Press the seam open.
Sew on another strip, press it open. Repeat until you have covered up the white block.
It should look something like this when you are finished.
The back will look like this.