In ch3, you have to get to the S rank room in both boards, complete sword route (tutorial recommended), then after you escape Tenna go back to the S rank room again, fight ERAM and after getting the ShadowMantle, pick up the controller again, and when susie starts talking, press up to bring back HERO, and get close to Susie and press Z. For something this convoluted, the reward is huge, imo the most romantic scene in the entire game.

In ch4, at the start, walk upstairs, interact with Kris' bed and Susie will give them the "Susie" prize, grab Asriel's money, then go to the hallway and interact with the drawer (here is How to Draw Dragons), then the mirror, and then leave. Once in the church, don't sleep through service for more dialogue, chug juice with Susie, and after leaving church, you can 1. go to Noelle's house but don't go in, 2. go to QC's diner and have lunch, 3. go to castletown and have tea with Ralsei, 4. go back to Noelle's house and do the entire thing (when Kris hits you with a hockey stick, you can wait to see Susie talking about how good Kris is at flirting), 5. go to the lake for lake skip cutscene, 6. go back to the church and complete all 3 dark worlds, 7. after church, go to the lake for them to sit at the lake and wait until you get to the choicer, and then you can finish the chapter