Introduction:

Use reflexive pronouns if the object pronoun is the same as the subject pronoun. (I/myself, you/yourself, he/himself etc...)
Remember to use by myself / by herself etc... When the action is absolutely alone. Compare: I did it by myself. I did it myself.
Remember it is possible to use yourself and yourselves. Compare: You did it yourself. / You did it yourselves.

Structure:

(1st person singular) myself, (2d person singular) yourself, (3d person singular) himself, (3d person singular) herself, (3d person singular) itself, (1st person plural) ourselves, (2d person plural) yourselves, (3d person plural) themselves

Examples:

I did it myself.
She does it herself.
They went by themselves.
Does she live by herself?
Remember: Using by means without anyone else. I live by myself.