That is literally the million dollar question, isn't it?......I'm hearing Phil saying 😂
And it really is, because the one who can design and create a pattern and make a bag and sell 1000 copies of it with $10,000/piece will become eventually a millionaire 😆
Usually when designing any bag you start from the parts that create the general shape, the skeleton of that bag, the immovable ones which, usually, are the front and the back sides and sometimes the base.
Then, based on the perimeter of these steady parts you need to design the gusset whose perimeter will have it matching that of the fixed parts.
Now, that's simple in theory but how do you actually get that, you will ask...
There are two methods I know.
1. The traditional one, used by all modelists: walking the pattern to match notches, and walking which pattern part on which pattern part depends on the style of bag you're designing. I find this method prone to errors.
2. The Armitage method: matching the stitching holes numbers. This is my favourite method because it 'compels' things to match up nicely based on the same number of holes both on the large, steady parts (which should be designed first in this method) and the gusset. For example, if the front of your bag results in 100 holes, you shall trust that your gusset should have 100 holes also, whether you believe anything else. I'm not in anyway sure that Nigel has come up the first with this method, I just know the method from him.
I do also feel that professional pattern making, much like Freemasonry, is covered in some kind of secrecy, or at least discretion, because it really stands as the head of all things when it comes to making high-end leather goods.
But good design in general is at the intersection of a few skills of which some are learnable, while others are inborn. If one has not enough of the inborn skills (volumetric and abstract thinking, even some psychological traits I would add) he can't be a good designer and lot of effort should be put in acquiring these.