My studio's chair was kinda old and the seat was flat, so I repaired it and turned it into a tres chic element in my studio. I started taking the pics after dismantling the chair, sorry for the missing pic of the chair before the makeover.

Step 1: Find Your Victim

Sorry, I started dismantling the chair before taking the picture.

Step 2: Dismantle the Chair and Clean.

If you want to paint the plastic parts as I did you must deeply clean all the parts, so the paint sticks well to the plastic.

*Ask your cat for help, mine helped supervising all the process

Step 3: Add Primer

Add primer to the plastic to ensure that the paint stays where you want.

Step 4: Paint

Paint and let dry. It's about 18 hours so the smell of the spray goes completely away.

Step 5: Now the Cushion

-Remove the staples. I did it just with one side for making it easier.
-Take some stuffing material, I got mine from a crafts&fabric store. (and it's kitty aproved)
-Cut the stuff by the size of the chair
-Fill the cushions and re-staple.
-Get your new fabric and cover the cushion with it. Staple. (sorry, no pic)

Step 6: Assemble

-Get all your parts and put them together again.