I have lyrics of two verses below the staves.
Now I want the second verse to get the fortissimo (ff) dynamic. Now how do I do that? How is that written down in sheet music?
The most common way to notate that would be f-ff (or "whatever the first dynamic is"-ff). The idea is that the first time you would play at the first dynamic and the second time you would play the second dynamic.
I would advise if there are any other modifications to the verse like crescendos, decrescendos, and accents the verses be separate so the texture change is clear.
2nd time ff.
2nd × ff.
Or, even better, the same in italian, which I think would be
2a volta ff