This walkthrough is split by character, and then further split by endings that can be obtained. To obtain all images, follow all indicated routes; simply save at a branching decision that will affect how the game ends. Of course, you can also choose to skip to the happy end if you don't care to obtain all images.
If you choose to play blindly, keep in mind that the endings are dictated by affection levels: low affection leads to a bad end, decent affection leads to a tragic love end, and max affection leads to a happy end. Whether or not an option increases affection is indicated by a little "up" icon that pops up after selection. If no icon pops up, no affection is gained.
Occasionally, a screen showing a TV in a dark room will come up, and you will receive the option to "watch" or "don't watch." Every time you see this in a route for the first time, choose to watch it to view a short skit in which the little birds on board (known as Hiyokos) group up the characters in silly interview sessions.
To begin the story, first choose a protagonist to play as, then choose a partner that she will be spending time with. The first recommended partner is Kakeru Yuiga. Though you can start with any partner that isn't locked, the guys marked with a "recommended" tag should be prioritized, as they will usually introduce story elements that are otherwise glazed over in other routes.