How does she intend to accomplish this plan?

L. Macbeth says that she will drug the drink of both the guards while Duncan sleeps. Then Macbeth will kill the king and smear the sleeping guards with Duncan's blood and deposit the bloody dagger beside them. When Duncan is found dead, L Macbeth and Macbeth will pretend that they are shocked and horrified that Duncan has been killed in their castle. People will think the guards did it or, at the very least, someone paid the guards to do it. Macbeth will, seemingly out of revenge for the king, kill the guards before they can speak.