Complex sentence: Meanwhile, the beautiful but obnoxious Patrice Johnson (Christina Applegate) is so sure she'll be named prom queen, she's arranged for an all-night celebration with her boyfriend Roger (Matthew Perry), whom she is having on a string.
Split into simpler sentences: Meanwhile, the beautiful but obnoxious Patrice Johnson (Christina Applegate) is confident she'll be named prom queen. To that end, she's arranged for an all-night celebration with her boyfriend Roger (Matthew Perry), whom she is having on a string.