by this, or the present, declaration, action, document, etc.; by means of this; as a result of this: I hereby resign as president of the class. Obsolete.

Why do we use hereby?

Use the adverb hereby to mean "as a result of what I'm saying right now." For example, your bus driver might announce, "All cellphones on the bus must hereby be turned off and put away." The word hereby is especially useful for people who are issuing proclamations or reading from formal documents.

How do you use hereby correctly?

You use hereby when officially or formally saying what you are doing. I hereby sentence you for life after all the charges against you have been proven true. You are hereby appointed Sub-Lieutenant RNVR of HMS Tartar. I hereby give you the authority to sign all or any contracts on my behalf.