Unlocking all the events is quite challenging as the game doesn't provide any hints to attain specific events, which has to be deliberate to make the game challenging seeing as implementing a few sentences to serve as hints is seemingly not very difficult, pretty stupid as the game has quite a few bugs.
You basically have to satisfy specific conditions to unlock certain events, and you may lose the opportunity to unlock certain events after progressing the game to a certain point.
Maybe the developer had multiple redundant playthroughs in mind for us which actually serves to leave a bad impression of the game after completely finishing it. And seeing as there is no official walkthrough your best bet is to download the complete save file generated through the save editor from this thread if you want a complete gallery.
The game itself, plot-/artwise, is generally good, but factoring in the troublesome process of unlocking the HCG, which is the entire point of any eroge, and the fact that I have to resort to cheating to unlock them makes this a bad game.
Perhaps in future revisions, if there are any, it'll reach its potential