Best bet is to
check Reddit after new episodes air (they air on Fridays, so check Saturday). There's a guy called NederSubs that posts English subs pretty quickly. As for previous seasons, there are torrents with subs very easily available. Dusan is right, it used to be a huge pain to find them, but now the show is popular enough that they're easy to find as long as you know where to look.
Anyway, new season brilliant as usual. It started off slow for me, but the first couple episodes were really just tying up all the loose ends from the end of last season. New characters are great (Enzo and Valerio) and it will be interesting to see how they develop. I have a couple of hunches/theories, but this show always ends up surprising.