Question about Decoupage

I am one of those people that wants exact wording on some things especially crafts I am not real familiar with so having said that when instructions say use white glue diluted with water at a ratio of 3 parts glue to 1 part water are they talking about just plain white Elmers glue? One place I read about this said wood glue from a hardware store. Please is it just white glue you buy for school supplies?

I used to have some Christmas ornaments I made a zillion years ago in the Girl Scouts where we decoupaged material onto styrofoam balls. I loved those and want to make some more. I also would like to decoupage the light switch covers for my house. I see them in stores and think I could do that, I would also like to make some holiday ones to switch out.

I know it is probably a dumb question but I want it spelled out. LOL