Adding tweaks to the lock screen. Where do you start?

I'm trying to add a second clock to the lock screen with flex, but I'm not sure what needs to be edited to change things on the lock screen. I looked in the clock app but I don't really know what I'm looking for.

I know it can be done because there are flex tweaks to remove the big clock, so I figure it should be possible to add one as well.

Any ideas?