Cut out a square of the block size you intend to make plus an inch or two. For example, my block will be around 12” finished, so I’ve cut out a 14” block to start. Then you’ll cut strips of fabric to insert. The width can be anything. Here I’ve cut 1” , 2” and 3” strips of fabric. You’ll only need 2 strips and they need to be at least a couple of inches longer than your block - 16” long in my example.
Place the strips over the block in a pleasing orientation. This is where you decide how you want the finished block to look. Once you’ve place your strips, remove the first strip and set it aside. We’ll only sew on one strip for now. Place the strip over the spot you want it to be and pin it in place w/ the right-sides facing. Then go ahead and sew it into place right over the block using a ¼” seam allowance. Try not to stretch the seam at this point. It'll stretch easily since it's probably cut closer to the bias and on an angle)
Now you’ll cut along that seam allowance. Line up the ruler w/ the ¼” seam allowance and cut the larger 14” block.
Using your iron, Press the strip flat and then line up the cut portion of the 14” block piece (the piece you just cut off) right sides facing and sew another ¼” seam. Press seams flat. Press the block open. It should look like this.
Next you’ll place the second strip on your block and repeat the process.If your strip is wider, you may wish to place the larger portion of the cut block at an angle to create an insertion strip with uneven width. If the block looks a little wonky at this point, that's ok. We are going to trim it later. Finally, trim your block down to the desired size. Here i've trimmed by finished block to 12". And here is my finished block all trimmed up and looking perfect. And here is one I've made using the quiltcon colors: