How can i add closed alpha version to a already public app on playstore

I already managed to use closed alpha & open beta test before, but now since i´ve gone public i want to add new updates to closed-alpha for testing before i push them into the world.

My questions now are:

  1. how to make the correct closed-alpha version upload (im afraid doing something wrong to the public version)
  2. and how to find the download. will be both versions avaliable on playstore ?

thanks in advance