When you want something done in a straightforward and easy way without unnecessary complications, extras, or complexities, you want it done simply.
The adverb simply comes from the word simple, meaning "clear, easy, or uncomplicated." A recipe written simply uses basic steps that are easy to follow, while a room simply decorated might have just a table and chairs without any extra decorations or unnecessary furniture. Simply can also be used in a way similar to the word only as in "I'm simply asking a question."