Here is the pattern for it.
A to B = C to D = A to D = B to C = 6 inches. Leave 0.5 inch hem allowance at the top and 0.25 inch seam allowance at the bottom and the sides.
Cut four pieces of fabric using this pattern. Fabric type and color are your choice. I used a dark green silk cotton fabric and a blue-green polycotton fabric. I did a little embroidery on one side of the dark green fabric and I stitched a pink ribbon bow on one side of the blue-green fabric. (Sorry, you won't find it in this picture. I stitched it at a later stage. However, you need it to sew any embellishment or embroidery at this stage.)
Match the first set of fabric with right sides facing each other, and sew the seams on the sides and the bottom. Leave the top side open.
Cut the corners at the bottom to get that curvy shape when you turn it inside out.
Turn it inside out and you have the first bag.
Repeat steps 2 and 3 for the other set of fabric. The second bag is also ready. Do not turn it inside out.
Fold 0.5 inch for hem at the top of both the bags and pin.
Press the folds with your thumb, remove the pins, and iron the folds into place. Insert the second bag into the first bag with wrong sides facing each other.
Cut three straight pieces measuring 11.5 inches (length) by 1 inch (width) from a matching fabric (I used the blue-green fabric) for straps.
Now you will have three fabric tubes.
Repeat step 7 to make the other strap.
Remove each pin from the braided straps and secure the corners of the braid with strong stitches.
Place the straps 1.5 inches from the side seams and sandwich the strap corners between the hemlines at the top of the two bags and sew them in place.
You are done! And here is (are) the finished bag (bags).
Turn it inside out and you have another bag with a completely different style. (You will see the ribbon bow here which I attached before step 7.)
You can make this reversible bag and use it for two different occasions.
I would like to read your valuable comments for this tutorial. Please let me know if any of the step is not clear to you.