Now let's do the cushion ridges. You are going to have to do the ridges one by one unfortunately. Select a column of polys as shown and extrude it by 2.5 or whatever looks good to you. Move to the next column and repeat. Don't forget the two wider ridges on either side. There might be a little overlapping geometry here and there but that's ok because it will look ok when smoothed. When you are done, smooth it and see how it looks. Adjust if necessary.
Well it looks good but the sides look like they need to bulge out more so make the cuts shown in green. Now move the edges outwards to give more of a bulge.
This picture shows how the chair looks before and after the bulge has been increased.
OK, let's move onto the bottom part of the seat now. The seat bottom has a slightly different shape than the other cushion. Reshape it to be something like shown in the picture.