Grab your old t-shirt, the larger the better. Cut off the bottom band and stretch it out (this stops it fraying).
Cut out your design as shown by the red dashes. Try to get as much fabric as possible.
Hem the top , and then sew where shown in green. Remember to sew wrong sides together then turn it right side out after. To add extra strength, first sew a line of small running stitches, then sew a line of small zigzag stitch next to it and trim close.
Take your bottom band and cut where shown in red to give you two straps. The bigger the t-shirt was the better - I used a different XL t-shirt in my version shown above.
Sew your straps to the inside of the bag. If you use the cross pattern shown then your straps will have extra strength - match your thread to your bag colour and it won't be overly obvious.