I’m also a bit confused by this. Do you mean that you wish to learn the program? Or do you want something that will teach you pattern making and the program?
If it’s the 2nd one, the beauty and versatility of the program is that one can use any pattern making book to learn from and create the patterns in Seamly2D, and you are not bound inside the boundaries of only making garment patterns, but also quilting designs, bags, shoes, etc… I’ve even used it to design templates for boxes with windows for cupcakes.
I am of the opinion that the most important part to learn is to use the program. As you go, you will need to find solutions on how to do something, and this is where you look through the tools to find the best tool to do the job.
As soon as you pick up a tool, you get a small screen help in the bottom left of the screen that will tell you what to do next, so keep an eye on this space when you’re using a tool for the 1st time.
Here are some very old (2017) pdf’s that I created while I was learning the program. The Working with the Tools pdf is a 2-page quick reference on the tools that can be very handy to keep on hand while learning.
As @Douglas has said, there are some major changes being made to the software, so it’s not really feasible to do a new manual at this point, although I do agree that it is necessary, because there are so many amazing things being introduced at this point.