Clear stage 6 boss to unlock boxes in test stage. Enter the test stage. Move to Ultimate Reimu or Red eyes Sakuya. Press shot key to interact. Pres up/down to select bgm. Press shot key to confirm. 

Go to option. Change hitbox opacity to 0. Enter test stage. Press 0 to zoom in. Press 6. Change camera view to back view. Move up and move down to see skirt lifting. Press 2. Change Camera view to bottom view. See panty from bottom.