It looks like you’re pretty close. I probably would have started with a cube. Because it’s usually better to start with less detail rather than more. And from the photo, it looks like the ends of the backrest are a bit flat rather than completely rounded over like starting with a cylinder gives you.
Start with a cube and get the top left corner from n the correct place at the end of one vertical post. I edit mode grab the bottom face and pull it down and slightly forward so you have a skewed rectangle matching the slope of the back.
Now grab the side face and move it over to the other post. Now you only have 4 vertices on each end of the backrest to adjust to get the alignment right.
Now that the basic form is there start edging details: vertical edge loops to add the contour between posts, bevel the edges with only one or two subdivisions to make them more rounded and soft looking. Once this level of detail is done, subdivide and/or sculpt it to add the asymmetrical details of a soft cushion.