This actually doesn’t require as much upper body as it might look. This is more like a party trick that *looks* cool and complicated, but it’s actually not bad once you get over the fear of flipping upside down.
Keep your shoulders integrated into the torso. You can either wrap your hands around both sides of the hammock or put you thumb and index finger around one side with the remaining fingers wrapping around the other.
Use your core strength to lift the legs up and over. Try not to kick up (but if you need the booster until you build the strenght, you can.)
In terms of difficulty, this is one of those cool tricks that *looks* cool, but it’s not hard. A large part of it is getting over the fear that you’re going to die. 🙂
To practice lowering down, see if you can keep your legs lift as you unroll the torso and work to straighten the arms. The strength for this builds pretty quickly if you do this even just once a week.