First, Macbeth says, "If it were done when 'tis done, then 'twere well / It were done quickly." In other words, if simply killing Duncan would conclude the whole affair and render Macbeth king, then he would go ahead and do it now. However, a lot will have to happen in between the murder and his coronation: he will have to lie to his friends, act as though he is aggrieved and shocked by the crime, and perhaps even engage in some political jostling with Duncan's sons.
Next, he expresses his knowledge that the consequences of this murder will have to be resolved. The murder itself will not neatly conclude by itself; otherwise, he would be willing to risk everything, including his soul's time in the afterlife, to get the murder finished.
Then, he suggests that he will "teach / Bloody instructions" that could "return / To plague th' inventor." What he means is that, in committing such a violent crime, he will inadvertently show others that one can commit such a crime and get away with it. If he kills Duncan, then it stands to reason that someone could turn around and kill him next!
Next, he considers that he is both Duncan's "kinsman," or family member, and "subject," as the king ranks above him. Further, he is Duncan's "host," which means that he should be fighting to keep anyone wishing Duncan harm at bay, not considering murder himself.
After this, he reflects on Duncan's virtues and his goodness as a king, saying that he has been "So clear in his great office." Duncan has been humble and honest, so people everywhere will grieve for his loss.