Start by making your templates. You'll want one for the full shape of your bag and one for just the flap.
Use the templates to cut out the fabric for the flap and front side of your bag.
These are the two pieces you need.
Now tape the template together and use this cut cut out the full shape for the back of the bag.
Now cut a strip of fabric long enough to stretch around the diameter of the flap.
I made my own piping by taking plastic string, putting it in the middle of the fabric and pinning in place.
Sew the string tightly in place, removing the pins as you go.
Lay the flap on top of the back of the bag, wrong sides facing out.
Place the piping around the edge, so that the rough side is facing out, and the piece with the plastic is just on the inside of the fabric.
Sew in place.
Fold right side out to check that you have a neat edge.
Your bag should now look like this.
Now attach the front pat of the bag using the same technique, adding pipping around the edges.
Your bag should now look like this. Snip the pipping where it overlaps and stitch up the open edge.
Now use the same half template to cut two pieces of silky lining fabric.
Pin together, wrong sides facing out.
Sew around the edges.
Leaving the top edge unstitched.
Fold the flap inside out.
Pin the top edges of one side to the lining of the flap and sew in place. Fold right way out and then sew the other side of the lining in place.
Attach a twist lock closure to keep the bag closed.
Attach a small loop of fabric to either side of the bag and then clip on a chain handle.