May I suggest, there are numerous different ways to do an effective FF and they all depend on numerous different things.
First, FF is an acoustical phenomenon and very much depends on music texture, voicing, registers, etc. Say, for something like beginning of Beethoven's Hammerklavier, or 2nd movement of Op.101 it is enough to put very little effort for huge sonorities, while something with much "thinner" texture will require completely different approach.
Second, as any dynamics, FF can be perceived as such only in a musical context and only in comparison with previous (and following) dynamics. As such, it is not enough to know how to play effective FF, but even more important, how to make effective pp (even more important, how to make effective pppp--next to it a mere MF can be thunderous).
Third, very often FF is not a sheer loudness, but rather function of musical intensity. That is, often a phrase played pp, but with great intensity will 'speak' more than any banging FFF.
Last, not the least, all of the above will greatly depend on the style. Say, FF of Bach, Mozart, Lizst, Chopin, Brahms, Prokofiev, or Rachmaninov, will all be very different not only in musical meaning, but also in physical execution.
To summarize, one really would need to find a good and experienced teacher to help to discover all those numerous ways, possibilities, and executions of really effective fortissimo.
Hope it helps.
Best, M