Cut every piece that you need...

the big one 3 times

All together, instead the leather you can use some regular fabric you want

I made a nice embroidered line on the leather.

Pin the fabric together and sew

watch out!! try to get the half circle as an little bit smaller as your piece of leather. When you did, sew the leather on it with an other ebroidery stich

sew the back site on the bag inside out

The lining, I made a small pocket for it.

sew both sides inside out on each other

The closure. sew together, put inside out, and stitch

sew it on the back site of your bag

Put the inside in the outside

Close it with a piece of bias band or make one with fabric as i did

And finaly the straps, then you are done. I hope you will enjoy it