You begin the process by buying urine control powder from the shady guy outside the shop. Next you have to choose to keep running alone with Melinda at 11 am and complimenting her (on the days they run which I believe are Tues, Thur and Sat, always making sure you have the urine control powder beforehand). The beginning part of the blackmail scenes should trigger if you have already completed the scenes for getting Uncle Will drunk & visiting Melinda at 14:00 after doing so (those scenes progress until the sex scenes in the laundry room are all played out, if you don't know what scenes I'm talking about you need to take a peek at the walkthrough then because there's too many triggers needed to get to those scenes that I can't get into detail about here.)
Lastly but very important, the blackmail scenes won't fully continue until AFTER you choose to let Cynthia go. Once you have resolved the blackmail situation the night time sex scene will be available when you repeat the dinner date a few times.