You have to have a decent amount of hearts, 4 I think and I THINK you need her arousal to be decently high as well. Then you need a pink candy that you can buy from Aki's shop that randomly appears around the map. When you get home don't make dinner immediately, go to the bathroom and select the "search bathroom" option until you see a cockroach. After that feed her some milk with the pink candy and that'll trigger her going to the bathroom to masturbate if it doesn't keep feeding her milk and then talking to her to see if it does. Masturbate while watching her until the cockroach shows up, at which point you'll march inside and the scene will begin. If you want to trigger it again you'll sadly have to do all the steps mentioned above again but yeah.
(You also need the Deluxe Edition but that's a no brainer)