I am considering a new saddle - a Brooks or a Selle San Marco but haven't a clue about how to measure for a saddle.

I have hunted other posts and know the difference between sit bones and pelvic bones. I just need to know how to measure sit bones.. I know that awhile ago I ran across a post that gave the how to's, but I can't find it. Do ya' just stand on your head with a tape measure in your hands and hope for the best ?

Thanks for your help!