Belly-down is the easiest and most stable attitude, which is why that's the first attitude taught in skydiving.

The other common attitudes are head-down vertical, feet-first vertical, "sitting", and back-flying. The first three have a much faster fall-rate than belly-down, and are less stable. Backflying isn't quite such a difference in fall-rate, but is still harder to learn, and less stable.

How easy is it to learn? Well, unless you've skydived before, then you get around a minute to learn from scratch. Good luck to that!

It's easy if you know what you are doing. It's not a steep learning curve but there is one.