Make your cuts. For each bowl cozy, you will need: 2 x 10x10" fabric pieces and 2 x 10x10" batting pieces.
Use the marking pen and a clear ruler to draw four lines on each fabric piece: 1 line drawn vertically through the center of the piece; 1 line drawn horizontally through the center of the piece; 1 line connecting the top left corner with the bottom right corner; 1 line connecting the bottom left corner with the top right corner.
When the markings are made, place the fabric piece face down on one of the batting pieces, carefully aligning all corners and outer edges. Clip the pieces together. Stitch along each of the drawn lines, and then use a damp cloth to remove the marked lines. Repeat with the remaining fabric/batting pieces.
Fold one of the pieces in half along one of the stitched center lines so that the batting side is facing out. Using a ruler and the marking pen, measure on the top edge of the folded pieces 1" from the center line of stitching and make a mark. Now, measure 2" down from the top edge along the center line of stitching; mark. Connect these marks with a line, as shown below. Repeat these steps along the bottom edge of the piece. Stitch along these lines, and then trim the seam to 1/8".
Next, fold the piece along the other center line of stitching. Measure and make marks and the stitch and trim in the way detailed in the previous step.
Fold the stitched pieces in half, making sure the two darts at the center of the front and back top edge are aligned. Use the corner template (available for download in the project post) and the marking pen to round off each corner. Use shears to trim away the corner points.
Repeat the previous steps to prepare the other bowl cozy body piece.
Place the prepared pieces together, right sides facing, so one piece is nestled inside the other. Make sure to align each of the side darts and the stitched lines of each of the pieces. Clip together.
Using a 1/4" seam, sew along the outer edge of the pieces, leaving a 2.5" opening for closing. Leaving the opening seam untrimmed, trim the rest of the seam to 1/8". Turn the piece right side out through the opening, using a seam creasing tool to ensure all of the edges are crisp.
Press the piece along the stitched outer seam, tucking the unstitched seam toward the inside of the piece. Clip the opening shut. Topstitch 1/8" from the edge along the outer perimeter, closing the opening as you go.