its not hard really, just need patience!! its far easier if you take the back part off so you are just working with the base..........

This is just the way i did it, there may be better ways! But pull the leg support bar out as far as it will go, then pull the metal clips out from underneath and you should be able to pull the support bar off its two metal bars........

Then by being careful, you will see that there is metal rods going down the sides of the main cushion, these are just hooked under metal tabs. You may have to prise these a little with a small screwdriver. Once you have done that, fold the base of the seat over (you dont need to remove it fully)

You will then see that the side bolsters are held on in a similar way, being careful, i managed to peel them back and remove the foam. Then put the new piece in, it is very fiddly though and getting the foam over the metal bar that makes the bolster solid is hard, but plenty of waggling and you're there!!

See these pics for an idea.........





The difference!