She gives a clear and exhaustive example of each of the titles and their families and how they were addressed. Since I cannot improve upon perfection, I won't try, so I'll simply pass along the source in case you struggle with it.
However, there has been some discussion as to whether it was correct by the Regency to use Marquess or Marquis. I prefer to use Marquis. It just looks better to me—maybe because it looks French. Debrett's 1802 version, and most of the peerage books published at that time, use Marquis. Debrett lists the plural as Marquisses. However, by 1828, Debrett changed the spelling to Marquess. The change may have resulted to popular use as in the way dictionaries keep up with modern terms and meanings, or Debrett may have caused the change. I do not have an original statement as to the wording when Wellington was created a Marquess so do not know how it was spelled, but The Royal Kalendar of 1815 says Marquises.