We unfortunately won't be writing a step-by-step guide for Reflections, but we can share a few general tips that apply to every route.
All affection points are carried over from the common route section.
To open a character's route, you only need 1 affection point with them.
The final "Where should I go?" event in the common route will raise your affection with the character you meet, even though there's no choice.
It's best to keep a save file at the very first choice relevant to the character you're pursuing, within the common route. It's easy to lock yourself into an ending if you're saving within a character's route itself.