A testing methodology is developed by people with a lot of subject matter expertise. If you need to ask this question, I respectfully suggest that it would be in your interest to seek out a good book and/or purchase a properly developed methodology. There are many sources on this forum.
If you refer to a testing strategy, then you may be able to check out previous threads on how this should be developed, when, where, and why.
Thanks very much for your direction. Actually what I need know is not the content or what testing methodology is. I think I have enough information regarding subject matter, and actually we've our own test methodology but we've never attempted documenting it. Since it's my first attempt, it would be really helpful if I can see how such document should be organized. Again I appreciate you advise.
If you've got a test methodology, then just get started, and tweak the format later.
I've seen all kinds of formats; most choose either a notebook or a centralized on-line repository. Put a heading on a page, like "NAMING CONVENTIONS", and write it down. Then move to the next topic, like "DEFECT MANAGEMENT".
Methodologies are normally a group of documents, not one document. Just break up what you've got into logical pieces. The hardest part of any large effort is to just make yourselves get started...
There's no "standard" for documenting test methodologies.