It's a pretty in depth process that even I who has played through it a dozen times, often have to refer to the walkthrough. It's in the Monastery section.
One thing I can add, that isn't in the walkthrough is how to replace the plugs. For each of the three, you have to peak in, see that they are masturbating, then sneak in and replace them. Both Heather and Bridget do it a couple of hours after dinner, sometimes Mira doesn't until around midnight. Though the last time I did it about a week ago, I actually caught Mira at 8 pm, it was the day I got the plugs, the day after we arrived.
Usually I accuse Antonius, but this last time I actually was able to say that the murderer ran away. I don't feel like I advanced Heather any differently than before, I have read her letter before, but somehow this time the option was there.
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The pot of food for the girls becomes available at 7 pm. It's down in the kitchen near the dead body. Heather gets in her room at 6 pm, Bridget and Agatha at 7 (maybe 8 I forget), then Mira an hour or two later.
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Anyways after you have the murder portion solved, you will still have a lot of work left on the girls. Usually I will make a save, then make sure I actually can get Antonius arrested, or now successfully tell William they got away, just to make sure that portion is handled. Then reload my save so I have ample time to switch the plugs and take the virginities.
All of the girl corruption is only available from 6 pm to about 10 pm, so a lot of sleeping until evening. (Make sure you have a lot of bread before you go to the monastery, you can eat there in the dining room, but it is only available at the same time the girls are.) Skipping an hour to get the food pot. Painfully slowly get Bridgets trust and play with Heather's butt way more than you should have to, go to sleep. Then repeat that day for about a week, slowly advancing.
Time does get real wonky regarding the girls, like you will stay the night with one of them at 8 pm, you wake up at a strange hour like 6 pm, or midnight, but when you talk to the girls they say things like "I already ate" or "It's too late, come back tomorrow." I actually avoid staying with them to avoid that.
In your spare time trying take Heather and Bridget's virginity, it is worth noting that after you switch Mira's plug she is now finally corruptible by asking her for head.
You can't get head from Bridget until you pull out during in sex, where she then takes it to the face. Which she eats, and it opens up asking her for oral after asking her for encouragement.
