Sansha Mendan is overall a fairly simple game to play, but some things are obtuse or difficult to understand at first. I'll explain pretty much everything about the game here.
-A day-by-day walkthrough of this game is completely unnecessary, if you just consistently do events for all the girls at once without using the "wait" function too much, it is very easy to see all the content this game has to offer without missing anything. Focusing on one heroine is fine, but you'll need to restart from the beginning to get the harem ending which is unlocked only after corrupting every heroine all on one save file. The skip function makes this not a big deal, so don't worry about focusing on one heroine at a time.
-On the map screen there are images of the heroines' faces which you click to initiate an event with them. Full-color face image means it's a plot event which progresses the story. Dimmed-color face image means a minor scene which does not progress the story. You will not be able to see these minor events if you progress the story past them, but since they expend in-game time you'll want to quicksave before reading them to not lose out on any time (though there's more than enough time to not really worry about this - the time limit is 50 days.)
-Cumming inside or outside doesn't change the story, but if you want 100% CG collection, quicksave at the choice and go through both.
-A big theme of this game is threesomes. There are a lot of threesome scenes. But in order to see most of these scenes, you have to be careful about the corruption order for each pair of heroines. One girl must be corrupted before the other, or the threesome scenes for that pair will be locked out for the rest of the game. Order is below:
Aina/Yuuna: Corrupt Yuuna first (Do 3P scene before corrupting Aina)
Marika/Mifuyu: Corrupt Marika first (Do 3P scene before corrupting Mifuyu)
Hatsune/Narumi: Give Narumi all her rewards (Do 3P scene before corrupting Hatsune)
-There's a "strip" option in the menu which allows you to make the girls wear nothing but their underwear, among other things. Most of this underwear is never shown anywhere else in game, so be sure to use this option to see them if you want to. Additionally, the girls change what underwear they're wearing after each story sex scene (for a total of 4-5 pairs each), so keep that in mind.
-Hold enables/disables the presence of the menu on the bottom of the screen. Cut-in enables/disables the secondary CGs overlayed over the primary CGs (such as shots of the MC kissing the heroine). Face/bust/hip determines which part of the heroine's sprite is focused on when she is close to the protagonist.
-You can collect items whenever you feel like it. They unlock optional post-corruption scenes and have no impact anywhere else.