There are six individual heroine achievements and this combined one - the combined one requires those six AND Strea.
*** Spoiler - click to reveal ***She isn't accessible after floor 96, so you need to get her to max affinity + get her bed CG before then or rotate into NG+ for it. You might be able to do this if you do the side event on Floor 100 to rescue her, but I haven't tested it.
Get the heroines to rank 5 affinity, then get their bed CG which counts towards Memories. Either grind this out in combat by praising them, entering conversation with them when there's a yellow text bubble above their head, or completing their Hollow Area requests and using the garden in the Abandoned Area to grind affinity. Wait for orange bubbles, talk to your partner, spam space + 2 + left click and spin the mouse like a fucking madman and it might actually work sometimes. It's inconsistent. The conversations are incomprehensible and I haven't bothered figuring out the optimal route for each, but spamming 2 will get you the best results the majority of the time. Be warned, this is incredibly tedious.
If you stand by the fountain in the garden, you're guaranteed to have all the correct responses be "2", but triggering the orange bubbles there is inconsistent. The bench next to the fountain isn't quite as reliable for correct responses, but spamming 2 for everything that doesn't have "legend" in it works pretty well.
Also, you can buy them food in Arc Sophia, but this is incredibly slow. If you feel like setting up a macro it might work, but outside of that I wouldn't bother.
Extra part - once you've maxed out their affinity, you get easy access to their bed CGs, thanks to on-demand bridal carrying. Enter your private room at the inn with the partner you need the CG for, and get ready.
Exit the room, talk to the heroine, trigger the bridal carry, and enter the room. If you're spotted, you'll put the heroine down, and you've failed. Enter your room and try again. The room entry/exit is required for this, as it despawns all the other players in the inn, and you have a short window of time to enter and exit the inn before any of them enter the main door and embarrass your waifu.
If you've made it to the loading zone while still bridal carrying them, keep a movement key pressed down and do not release it until you've loaded into the bedroom, reached the bed, and the screen has started to fade out. Sometimes the game will drop the heroine for no reason here, and you'll have to start over.
Now you've got a standard conversation minigame, and if you beat it, you get the bed CG.
Enjoy.