Board Thread:Questions and Answers/@comment-27973255-20160313201444/@comment-25776693-20160416213018

If you want a simple version here it is (it's what i did at least):

1. Complete Neutral Route at least once.

2. Spare Flowey so he can give you the hint to True ending (dunno if really needed or not).

3. Don't kill anyone (duh)

4. Go on a date with Papyrus and Undyne

5. Make Asgore enter the room with the barrier (don't enter it yet).

6. Backtrack to the bridge

BOOM!! You're in.