First you need 5 hearts with Brooke, after that when you talk to her she will tell you about the beach. Next on sunny morning between 11:00 to 13:00 (check Brooke house if she is not there means she is at beach) Go to where Derek is if you face him take the road leading to right. When you reach the stairs “if Brooke is on the beach the rope is there“ you climb down, cut grass beach appears and scene starts.And what are the pre-requirements? I still don't see it. Thanks for your help.
On another note I will suggest you do this one after you have done Laura and shopie’s glory hole because when I did Brooke quest first the other two events just won’t trigger.