I found some cheap goat leather in the internet. In Spain there are some sites where one can find real bargains.
To start, I bought all the tools I was going to need to complete my project. A self-healing mat, a rotary cutter, a Teflon foot, hand and machine sewing needles special for leather, double sided mounting tape, and since I couldn't find the cold rubber tape Don was using, I substituted it for some surgical tape (made of clothing, it performs the function of preventing leather form stretching when sewn):
As Don recommended, I started by doing a mock up bag in felt. It proved to be helpful, because I changed the measures, and then used it as pattern to cut the lining and leather.
Then I made a zipper pocket and it bag, and on the opposite side, I sew a rectangle and divided it into two pockets for phone, etc.
The first thing I sew was the handles. I put some felt inside for them to be thicker. It took some essay and error to adjust my machine to make good stitches with the thick nylon thread I used, but I managed to do an acceptable job after all:
I sewed the two lateral pieces, stoping in every corner, then taking the thread to one side and knotting them up, since we cannot go backwards with the machine when sewing leather.
I measured the complete lenght of the bag opening and then finished the lining (with mitered corners) and its leather belt on top, which had to be the same lenght as the bag opening. I attached some magnetic fastener in the middle of this leather piece. I sewed one side at a time, previously applying double sided tape, and knotting the treads together after sewing each of the four sides:
Amd this is the final result. It is a lovely white leather bag, just as I had imagined it, but winter has come, and this project, planned in summer will have to wait in my wardrbe for the sunny months ahead: