Unlike what zdubidoc-ga has written, there is no real "secret" here.
However, Gucci or Parada haven't been where they're at if everyone
just knew how they make their bags. The components there are
innovative design (something you either have or not, and then maybe
have to copy); good training (not from a book or a tutorial, but from
several years of apprenticeship; and high quality material.
However, here are several steps, with which you can start.
Sites which provide some idea.
Making a Custom Made Leather Handbag
Lets make another custom handbag
Both of these link explain step-by-step, how he makes custom handbag.
However, this is of course not enough for someone who had never done
Tutorials and Distance Learning Methods
You can start here, but if you want to become a real designer, you'd
better consider a "real" school (from the next list).
Leather Connection Tutorials
The site sells three tutorials, and plans to issue one specialising only in bags.
Making Handbags: Retro, Chic, Luxurious
Ellen Goldstein-Lynch, Sarah Mullins, Nicole Malone
Making Leather Handbags and Other Stylish Accessories
by Ellen Goldstein-Lynch, Sarah Mullins, Nicole Malone
by Terence Terry, Caroline Darke
Hip Handbags : Creating & Embellishing 40 Great-Looking Bags
by Valerie Shrader
Shoe School - Video Tutorial - Bags - Belts & Accessories
Schools where you can learn the profession
If you're serious about becoming "like Gucci" in your quality,
learning to swim in correspondence, or learning to make handbags
without being in a professional studio - will not work.
An Italian fashion school. They offer a degree in fashion design, and
naturally also have focus on leather design.
Another Italian school, offering a one-year course in bag design and making.
Fashion School of Design - International Fashion Institute
A third Italian institute, offering a course in accessories.
University of Arts, London
The university offers numerous courses in leather design and making:
Fashion in Leather
Fashion in Leather II
Advanced Techniques for Leather Accessories
Box Work Handbags
Wallsall College of Arts and Technology
Offers several courses on leather design, as well as an apprenticeship.
1. intro to handbags
2. making leather handbags with michelle courtois
Workshops held in New York.
I hope this answered your question. Please contact me if you need any
clarification on this answer before you rate it.