I'm not an expert in fitting, but:
--Is your friend non-binary? Why are you using a menswear book for a woman? Women have different builds and often require bust darts and more shaping than men;
--Make the pattern in plain muslin, not a busy print, so it's possible to see clearly what's going on;
--Make only one change at a time;
--Make the full garment and close it pinned in the front;
--Take more photos from more angles.
This is a good fitting book. It's expensive but you can probably find a copy in a library or buy an older edition used.
Assuming the shoulder seam is supposed to be on the shoulder, it's too low. I would try pinning a large tuck to try to bring the seam up to the right position.
Personally if this was my first bag (and it sounds like it's yours?) I would get a book like this, muslin it in a stiff cotton, and make alterations/additional design choices from there. You can then cut up your muslin to use as your pattern for the leather.
I love love this method, though the book is hard to get: https://www.amazon.com/Building-Patterns-Architecture-Womens-Clothing/dp/1424343534
As far as I know Craftsy have a ton of videos from the author, and they are AMAZING!
I drafted a sloper that I use for my pattern making, but I wouldn’t recommend it as a beginner project. Start with a fitting shell to draft a sloper. If you only buy one commercial pattern, it will set you up for a basis for drafting.
https://www.amazon.com/Vogue-Patterns-V1004-Misses-Fitting/dp/B000TUVVNG
Since you’ll probably want long, sweeping lines, I think you’ll do best with a hip curve instead. One like this.
It might help to start with a fitting shell. Once you have that fit, use the excellent book you have to practice dart manipulation, etc. Then, when you’ve leveled up, try drafting a block from scratch.
https://www.amazon.com/Vogue-Patterns-V1004-Misses-Fitting/dp/B000TUVVNG
It's similar to a thick manila folder. It's not the same as posterboard or tagboard, which have a central core.
Here's an example. Sometimes it's beige on both sides, sometimes it's beige on one side and green on the other. https://www.amazon.com/Fashion-Design-Pattern-Weight-Journal/dp/B099549RCP/ref=sr\_1\_2?keywords=oak+tag+paper&qid=1637095900&qsid=135-6840255-8264628&sr=8-2&sres=B099549RCP%2CB001E69XTS%2CB0027667F2%2CB01N4NIVWW%2CB00PEFCPT0%2CB07...
It's hard to know what to recommend without knowing your background, what you are trying to do, and your budget.
Reducing or enlarging the size of a pattern is called grading. Grading a simple shape like an A-line skirt up or down a couple of sizes is not that difficult, but more complex shapes and moving up or down more sizes is much more challenging.
Professional Pattern Grading for Women's, Men's, and Children's Apparel by Jack Handford was highly recommended here. I've looked at it and it seemed straightforward. I suggested it to a friend who loves it. But she did have pattern making experience.
Many designers who are not pattern makers hire a pattern maker to make a pattern and grade it to create a range. You might want to consider that.
This one is used for armscyes. https://www.amazon.com/Lance-FC-1-French-Curve-9/dp/B000HFADRS/ref=sr_1_15?dchild=1&keywords=french+curve&qid=1631709015&sr=8-15
But would anyone be drafting a pattern without that basic tool? You need that curve, a hip curve, and a large L-square.
Medical paper. It’s thin enough to see through and great for drafting patterns.
Scotch magic tape. Not the shiny type but the matte type that disappears when you put it on paper. Get a heavy dispenser so you can tear off pieces with just one hand.
I hate metal rulers of any sort. I like the thick plastic ones that you can see through. Don’t invest in good ones until you know what you like.
Mechanical pencil. Even with an electric pencil sharpener, it just takes too much sharpening as you go for a regular pencil to be handy.
Make a muslin with your balance lines and grain lines clearly marked on it (or try gingham.) Adjust the fit on the muslin while wearing by either pinning out parts or slashing and pinning strips of muslin in the slashed area to add width or length. Mark the changes on your paper pattern. Then, repeat with a fresh muslin until you get it right.
If you don’t want to do all the drafting, try starting with a fitting shell.
Edit to add an example of a fitting shell. https://www.amazon.com/Vogue-Patterns-V1004-Misses-Fitting/dp/B000TUVVNG
Not drafting, but tailoring techniques: https://www.amazon.com/Couture-Sewing-Techniques-Claire-Shaeffer/dp/1600855040