This is a tutorial the other way round: I will not tell you what you should sew to make a certain thing, but what you can make of what you have got.
I'm often asked questions and will try to answer them here to share my experience, which you will not find in the maze of the Internet.

What to start with?
So, you've got an idea to sew a leather bag. Where to get the material? I had two ways at the beginning of my creative career: sad old leather coat I will never ever wear. And the second is to buy leather or trimmings. You must not be under delusion that a super-duper coloured leather in large pieces would come. All is much more prosaic. 5 killos of black and brown pieces of the size of a palm. Don't get upset and don't despair! Just change the direction of your mind. Cut out a template out of cardboard. Cut piles of pieces with a silver pen, which is easily erased. Buy fabric which will serve as a base for your bag, it should be thick, not stretchy, or just use thinner leather for the same purpose.

Material and tools

• scissors (or knife, if available)
• glue
• silver pen
• hammer
• threads, better nylon and brush

The glue is obligatory! Rubber or Moment. I will explain in detail why the glue. Most of us have a simple sewing machine with one movable ripple device and a stationary presser foot. If a detail is not glued, it will simply move out while being stitched. Leather doesn't like any extra perforation.
Iti's clear where to buy thread, scissors and fabric. Rubber glue can be bought in shoe repair shops. Any shoemaker can tell where he buys glue — they use it to glue locks when when repairing. I write all this in such detail because 90% say they don't have such goods in town :) There, you just do not know about them :) Here is the glue. It has only one disadvantage — does not store well, gets dry, but is inexpensive.

How to make a pattern

Since this publication is mainly for beginners, then I will tell where I took patterns as a beginner.
I ripped my favorite old but obsolete bag. Tip: if a bag contains more than 5 parts, cut it in half, then take one half to rip it and glue on an A1 paper. Make the details align and symmetric.
When the pattern is dense, it is cut easier. Leave the second half as a sample how to assemblу the pieces. I started with simple patterns.

Despite the simplicity, even three parts are a great experience for your imagination.

And another simple pattern in the photo is the pattern of a pocket bag, it's just a square of size 26 by 26 cm, with rounded edges and a seam for the volume. A small detail, stitched bag visually looks smaller. And even if it has no side and bottom details, you can always stitch them.
The formula of this pattern is simple: the desired size of bags (for example, for A4 size) + 3 cm (lock with lining) + 0.8 cm seam allowance, if any. Sides: the desired size + 1.5 to 2 cm (to make a side to create volume) + 0,8 seam allowance.

When there is a base, it is much easier to experiment. Patterns from the Internet can not be checked and believed :) I made a bag of a middle size adorning it with a cusp for the handle. If you make a new pattern, remember about the allowances for seams and folds!

If you found the material, made the pattern, then start assembling.

Assembly of the bag
1) Before you cut all the details, they should be ironed. Preferably on a hard surface and with not very hot iron.
2) Duplicate. What is it for? What to duplicate with? I'm not going to tell you about special materials, use what you have. It is not necessary to duplicate leather, but if you have sheepskin or pigskin, which is thin and does not keep the shape, duplication is obligatory.
1) Vlieseline. Layer by layer, you achieve the desired thickness. I use then all sorts of application which are sewn to the duplicated parts of the bag, thereby I sew the vlieseline and it won't peel off ever. One disadvantage — you can't carry heavy things in this bag. Vlieseline bag with 7 layers and applique. The base of thin pigskin with a thickness of 0.4 mm, it is clear that will not keep its shape without vlieseline.

2) Dublirin for curtains. This bag will be on the frame. Disadvantage — if dublirin is bended, you will not improve it, but actually you do not need to fold a bag in half :) A tip: a dublirin pattern should not fall under machine sitches, or the seam is not smoothed.

3) Insulating material for construction. Bags are just weightless, very light. Here you will need the same rubber glue to duplicate. The bag can knead and fold, nothing is damaged.
One minus — it is not fond of weights, does not keep the shape. It is more suitable for small handbags like clutches.

Handles and straps
The good width is of 3 cm, it should be necessarily duplicated either with a top cord (sewing leather parts on it) or a usual flexible tape, inserting it inside a handle between the two layers of leather. Glue is all that you need, otherwise the handles will have bubbles.
Seams
In leather bags, seams are treated with a hammer, it is a kind of an iron. By the way, when connecting the parts, I glue them with rubber glue on the rigth side (one should not glue the velour or suede, glue is not removed from them) so that nothing moved away in the process of stitching. Then glue the seam from the inside and smooth with a hammer: the seam is no longer bristling.

A few words about this tool: it is good if it is a shoemaker's hammer with a round cap, but you can also use a regular one. Be careful! One careless punch makes a hole in leather.

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If there are small pieces of leather, make a kind o a quilt. To do this, glue the parts and stitch them. Then everything follows the idea of your pattern. And, of course, decorate it. There's no bad material. There's a new idea!

And — do not be afraid of the holes, spots and other imperfections, this is just a place where it is necessary to make a strap, stitching or other decorative element. Any imperfections on leather is a chance to create a masterpiece. That's what makes your bag unique! And remember — even the simple leather bag is a hundred times better than any leatherette.
Be patient, bags are not sewed fast. One can sew a lot of bags in different styles with one pattern.

I hope I have shared useful information and maybe someone has found the solution of a problem.
Don't be afraid of imperfections: try, experiment. A hobby that brings income is a good hobby :)