You use the adverb 'Hereby' to mean 'As a result of what I'm saying now'. For example, "All cellphones must, hereby, be switched off and put away."

Don't try to use it. It is old-fashioned, and rarely used EXCEPT in legal documents and formal public presentations. At a wedding, one customary form of words is "...I hereby pronounce you husband and wife." The full context would be something like "By the powers vested in me by the X Church and the State of Y, I hereby pronounce you husband and wife." In short, "Because I have the authority to marry you, you are now married."

My best guess is that it means "now". "I hereby declare I am innocent" means "I now declare I am innocent (in the presence of witnesses)". It is antiquated language that I don't hear very often anymore. In fact, I've never used it nor have I heard anyone use it outside of formal proceedings like a courtroom.

I found this example : We hereby certify that the following student, who holds a Korean passport, was a full time boarding student at .........