You have to push the lower half hard towards the back of the car. There is two hooks that hold it in place. One on each side. Push back and lift it up. Then do the same on the other side.
I just stand up in my convertible and use my shins to slide the seat rearward and out of the hooks. You'll have to squat down and use your hands to push. One good hard push should do it for the lower cushion.