[VRMN]

Ah, that's rather difficult. The problem is that the first season doesn't just deviate from the plot, it changes the order of events significantly.

For example, the last thing that happens before episode one of the second season in the manga is a trip on Sakuya's boat and this will more than likely be referred back to in the second episode (as it was in the corresponding chapters to those events). This is chapter 51 and 52 of the manga. It's episode 43 in the first season.

The first season has a lot of anime original material, but they more often moved to create their own timeline of existing events. Hinagiku doesn't show up in the manga until chapter 35 of the manga. She shows up in episode 4 of the first season in pretty much exactly the same way as she shows up in the manga with some slight modifications to the manga's rather abrupt ending.

So, honestly, you really have to read the manga to get everything. That said, the last major event to occur before the second season jumps in is episode 16 of the first season. Though, again, one really important event (that occurred earlier in the manga than those events) is in episodes 24 and 25...really you should probably read the manga through chapter 52 (this is near the end of Volume 5).

Personally, I find the first season worthwhile just on its comedic merits, but the plot of the second season expects that you've read the manga up until that point, even if you have seen season one. It should also be noted that it's unlikely that the second season will redo any events that the first season did on its own, even if it was really out of order. A lot of side stories were done in the first season that will probably not be repeated here, even though they chronologically happened after where the second season jumps in.