Well, through lewd action/special moves (depending on the game), you can make the girl gain 1 heart, up to 4 in total.i dont understand i need to get heart for these scene? If so how ? And once i have them how do these scene trigger?
When you reach one heart, she'll replace her next attack by a h-scene. If you let her do it, the scene is triggered, and she lose her heart. Block the 'attack', and she'll resume her normal attacks but keep the heart.
Increase her hearts again. Each 'level' will make her attempt to trigger a h-scene, and the scene depends on the number of hearts.
For example, the cowgirl scene correspond to 4 hearts. So you'll have to increase her number of hearts while blocking her lewd attacks until you reach 4 hearts.