Complex sentence: Stan, Kyle, Cartman, and Kenny are grouped together with Tweek, a jittery child, who suggests that the presentation should be on the "Underpants Gnomes", tiny gnomes that sneak into his house and steal his underpants.
Split into simpler sentences: Stan, Kyle, Cartman, and Kenny are grouped together with Tweek, a jittery child. Tweek suggests that the presentation be about the "Underpants Gnomes", tiny gnomes that sneak into his house and steal his underpants.