That requires a sail needle. It is a curved needle available at the fabric store. Purchase button thread in the color of the fabric.
Hi Debbie, here are a couple of videos for you to check out
A cushion cover for a child's car seat, or for a regular front or back seat in the vehicle?
Is it a bucket or bench seat?
Is the cover on the seat ripped, and you need to repair it?
do you want to sew an additional brand new cover for the seat cushion (to go over the existing one) ?