Autostart containers

I’m kind of new to this but I read the wiki and found the relevant section that is support to autostart a container.

      image: nginx
      restart: always`

However my problem is that I have already created the containers and they are already running. I have tried to enter my existing image name and name but it wont work. If I use new image names and names it will start a new container with defaults but my existing containers (with custom option) remain stopped. What am I doing wrong ?

Thanks so much for any help.