Seat: If it's a genuine leather seat, buy a cushion and backrest. If it's a cloth seat, buy a seat cover. Car decoration stores sell it and they will install it for you. You can also buy it in the store and install it yourself. It's very simple, just wrap the seat cover around the seat and tie it with a rope or buckle.
Steering wheel cover: You just need to install it yourself, wrap it on one side and then use force to install it.
Foot pad: You can lay it on your own inside the car.
Luggage cushion: self laid. Actually, it's easy to pretend on your own. If you think about it more, you'll be able to pretend. If you really can't install it, go to a regular car decoration store and find a master for 10-20 yuan. They will be willing to install it for you.