Cut two pieces of vinyl fabric for the bag outer measuring 12-1/2"x9" and two pieces of lining fabric measuring 12-1/2"x9".
Cut one piece of vinyl fabric for the outer bag flap measuring 12-1/2"x7-3/4" and one piece of lining fabric measuring 12-1/2"x7-3/4". Use a small bowl or plate to draw a curve at the bottom corners of the bag flap pieces and cut along the lines.
Sew the two pieces of vinyl fabric together for the bag outer along the two sides and bottom edge, with right sides together.
Sew the two pieces of lining fabric together for the bag lining along the two sides and bottom edge, with right sides together. Leave a 5” hole unsewn in the bottom edge.
Cut 1-1/2" squares in the corners of the bag outer and bag lining. Use the stitching lines as your measuring line.
Press the corners open with your fingers and sew to form the boxed corners.
Sew the two bag flap pieces together along the two sides and bottom edge, with right sides together. Clip the curved corners and trim the seam allowance.
Turn the bag flap to the right side and top stitch around the sides and bottom edge.
Pin the bag flap to the back of the bag outer with right sides facing, and sew.
Cut two 4" strips of vinyl fabric 6" wide. Finger press the outside edges of the strip towards the center and fold in half. Sew the doubled-over strip together and loop through each 1-1/2" rectangular bag rings. Pin to each side of the bag and sew.
With the bag outer the right way out and the bag lining inside out, insert the bag outer into the lining. Match the side seams, pin and sew around the top edge of the bag.
Pull the bag to the right side through the hole you left in the bottom of the lining. Topstitch around the top edge of the bag and sew the turning hole closed.
Insert a length of 1-1/2" wide cotton webbing over the adjustable strap slider’s center bar and sew the end in place.
Loop the other end of the webbing through one of the rectangular bag rings on the side of the bag. Pass the end back through the top of the adjustable strap slider and down through the second rectangular bag ring. Sew the end of the webbing in place to secure.
Sew a small piece of velcro to the bag flap and bag for closure.