How do you search for a file in Time Machine?

How do you do searches in Time Machine? (El Capitan)

Specifically, if you have know that files are in the backup stream somewhere, and you have a partial file name, or even a complete filename, how do you find which backup the file is in?

I could really use a step-by-step guide here.

My case: I know that somewhere in years of backups are several files with the word "Contract" in the name. Eg: Contract 2016 XYZ rewrite.pdf, and I want to find those files. I don't know the complete names nor the folder locations (but lets assume, if it is easier, that I can guess the folder location, which may or may not still exist). How do I find those files?

I've played with search in Time Machine for a few frustrating hours and I can't figure it out at all. The Search field shows nothing, even if I am currently browsing a folder that has one of those files right there and even if I type in the full name. When I type in the search field, some more criteria show up below the search field that let me specify 'name contains' and such. Sometimes this finds things, but only if I click on the 'this mac' button but that is a minefield of usability confusion.

I've used a third party tool to find backed up files, but then I have to go into Time Machine and search for specific backups one at a time. And every time I press the Up or Down arrow in the Star Wars display, it goes to the first (2014) or last (today) backup so seems useless.

The posts I read all say things like 'go to the folder containing the file'. How do I find that if I don't have the whole many-year history of my file system memorized?

Thanks - sorry if this sounds whiny. It has been very frustrating.