You can find basic instructions for Fast-90 right there in my post. Just take some time and read it with thought, maybe even play out the situation ingame and experiment with things, and it should become clearer. Wasting torpedos is much cheaper in SH4 than irl.
Wazoo's tutorial can be followed through pretty much just like in SH3. Yes the UI is slightly different, actually a lot better imho, but what matters is the TDC looking and working exactly the same.