The main thing Cyan used was his phone. First, Cyan used his phone to talk to Scarlet and ask her to get the pills, which is why Scarlet excused herself from chatting with me. Scarlet then took the pills and gave them to Cyan (He was wearing Orange's gloves at that time after stealing it, so that's why none of Cyan's fingerprints were found). Then, Cyan walked into Shamrocks room, switching his pills. At bedtime, Shamrock walked into his room to take his pills, not knowing his pills were switched. At morning, Shamrock awaked very late, still sleepy. Cyan then used his phone to ask Shamrock to go into the bathroom. This explains the wierd noises coming from Shamrock's room. When Shamrock entered the bathroom, Cyan also entered it, cutting Shamrocks head off. While Inverted Rainbow or some other cook was peparing the food, Cyan switched the food under the tray topper with Shamrock's head without the cook noticing. Also, i think Cyan already got White's body out of her grave by that time, but i don't know when that exactally happpend. After everyone found Shamrock's head under one of the tray toppers and went looking for the body, Cyan took the head and the body of Shamrock and throwed it somewhere we would not look. Probably some nearby river or a place where the killer hides bodies. Then Cyan joined the rest of the group without one of them noticing he was gone for some time. When an hour passed, Cyan and the others went back to the dining room, finding out Shamrock's head is missing. And that is how Cyan killed Shamrock.
Also, SlimeBlobFlynn, the reason i never was scared is because everyone that was is inactive.