There is all sorts of advice online about how to do things, but this piece on wikiHow on how to stop your beloved from marrying someone else is easily the best/worst I have ever seen.
If you were not able to contact her before hand, and you are sure that you want to proceed, find out the location of the wedding. Unless you have been invited, you will have to find out where the wedding ceremony is to be held, and the exact time. Ask family members or mutual friends if it doesn’t appear too suspicious, or perhaps check the wedding notices in the local newspaper.
I have never seen something so batshit crazy described in such a calm methodical way. And the photos! The caption for this one is “Enjoy life with your stolen bride or groom”: