I'm no expert, but it appear that if a tree was cut & left to dry for a long while (1 year?), then the bark would split & peel off almost on it's own! If you look at the photo, there is a cut edge perfectly straight (chain saw) & a jagged edge from natural ripping. I don't know how big the tree was or how it was left to dry, I'm guessing in fire wood sized chunks. But it wasn' split until the day we helped our friends. That's when I got it! It literally just fell off the logs.
As the wood aged, the bark simply peeled itself off & even flattened!
I have seen in the woods, were dead birch will rot, leaving a hollow bark cylinder. That would be a good way to harvest craft quality bark. Then all you would have to do is cut one side open, soak it good & then lay it flat with things weighing it down. Soaking birch bark will make it more flexible.
Some chunks did have stubborn inner bark that had to be pulled off. You can see this in the photo below.
The bark that I used for my star garland was pretty thick. Peeling it from a live tree would be very hard & tricky to get the full thickness. The stars I made are sturdy. But birch does have a tendency to crack along those pretty dashed lines. A bit of finesse is required when cutting, so they don't split or snap apart.