Imagine you had two hooks in the sky, about 12' apart.
It would be easy to hang a block from each, run a en endless loop of rope through each block and have the DS suspended by the loops (like slings).
After you've lifted your DS, you can flip it over in the slings. As you flip the DS, the loops would run through the blocks. (That's why you use a block and endless loop, instead of fixed slings).
First, nobody has endless loops lying around. So you take whatever strong rope you have and make a knot. Then you need to position the knots so that they start at the blocks and move away from them as you start rotating the boat.
Second, nobody has actual "skyhooks". But if you took your setup with the trees, and the two lines, you could lift each end of a horizontal 12' pole. Even though the ropes pull a little towards the trees, the pole enforces a fixed distance of 12'.
That means, you can use each end of this pole as a form of "skyhook" and attack a sling to it to lift your boat.
A little further thinking and you realize that a pole is overkill, a bit of rope (12') would do, preferably with an eye or ring at either end for easy attachments of lifting ropes and the load (aka the blocks with their slings).
With me? Or do you still need a diagram.
PS: I used 8:1 when lifting my DS from ceiling hooks... A 4:1 block and tackle plus a 2:1 cascade.