Since NTR seems to be a big on/off point for everyone here. Here's the guide for the True end.
Progress the Story normally. You can corrupt Amelia however you like. If you're me who plays for story and CG completion, you can get all the NTR CG's just fine
Once you're at chapter 5 (or even sooner? I can't confirm I got the true end after getting the normal end and playing the post-game). Confirmed as early as Chapter 3.
There are 3 Sub dungeons (puzzle dungeons no combat)
1. Indoors Stairs left of the Church
2. Upper right Corner of the Boy's Dorm
3. Right Tower on the outside FRONT of the School
Complete the 3 sub dungeons and you'll get the (2) True End CG's after you do the end game as normal.
Edit: If someone can confirm I got the order of 1 and 2 correct; I did it last night and don't remember if the order I listed is correct.
Edit 2: Both Order of Dungeon and Earliest Chapter Confirmed by
onore