Measure, cut, and press fabric. Cut two 16” x 18” pieces for the main sections of the bag, and two 4” x 28” pieces for the straps.
Take one strap piece and fold it in half, right sides together, down the long side.Sew a ¼” seam down the long side, leaving the strap ends open.
Place a large safety pin in one corner of the strap and push it through to turn it right side out. Press the seam flat with an iron. Sew a topstitch down the two long sides with a very narrow seam.
Repeat with the other strap.Lay the two main 16” x 18” pieces with right sides together, ensuring the sides are lined up perfectly.
Sew together the two longer sides and one shorter end, leaving one of the shorter ends open. Use a serger or zigzag stitch to finish the raw edges.
Turn the bag so the right sides are out. Fold the top raw edge over 1 inch towards the inside of the bag. Press. Fold over the edge one more time, about a half inch, to tuck the raw edges inside, and press.
Take the raw edges of the straps and place underneath the pressed fold at the top of the bag, about 3.5″ from each side seam. Pin in place. The space between the straps should be about 6.5”. Repeat with the other side.
Sew the top hem of the bag, making sure to sew down the straps as you go.
Pulling the straps up, topstitch around ¼” from the top of the bag making sure to sew the straps so they lay nicely up.