If I remember correctly, you need to let the lust bar to full itself, (it triggers a came in pants scene and all that). On the next day, (hoping that not triggers any other kind of scene) A scene of Miyuki discovering it on the laundry at the morning. Then, also you need to not trigger any other scene on that night. Then talk to sleep with miyuki, she'll ask you if you want to her to help you to relief yourself before bed.
If something happens in along the day that affect you in any way on that night that prevents you to ask to sleep with Miyuki normally, you need to repeat (Full the bar, wait to the next day, etc.)
To all this add that I don't remember if you need to unlock the 20 or 40 bar scene first. I might be wrong, but I think you don't need neither.
On the possitive, once you unlock it, you can ask her to relief you before bed anytime.
