How do I document a scientific discovery?

Scientists do this by publishing their research in scientific journals. First, you must have evidence of your discovery - either something someone else can observe as well or an experiment they can repeat. Then you want to read up on the field - find out what's already been done and where that kind of research is published. Then write up your paper, and take it to a local scientist in the same field and ask them to help you by reviewing it. It helps a lot to have reviewers before sending it to a journal. Then you can submit it (in the correct format) to a journal. Make sure you've backed up your discovery with evidence and you've cited other work where appropriate. The journal will either reject it outright or send it to one or more scientists who will review it and recommend the journal either accept it, accept it after more work has been done, or reject it.

I suppose you would have to have it all written down and then witnessed by several qualified scientists,

then publish it in a scientific journal. Contact the science department at your local university?

what was this scientific discovery?

tell us, and we'll tell you, if you really discovered something new!

Use a journal, and document as much as you can, research and experiment as much as you can. interview...etc.

I second the guy above me. What did you discover?

That is already well known