Look for alignments between Black's king and its pieces and see that it is lined up with its rook on c8. Since the king is in front of the rook rather than behind it, this calls for a skewer rather than a pin. But how? White can’t play Be6+ because then Black’s king takes the bishop. He can’t play Qg4+ because then Black plays QxQ. But one possibility remains on that light-squared diagonal: Qh3+, where the queen administers a skewer from safety. Black has nothing to do but move his king away from the rook (which, importantly, is loose). He can’t play the king to d8 to protect the rook because White attacks the square with his bishop. So next White will play QxR. The point is to see that there often are several ways to land a piece on the diagonal or other line needed for a skewer. You want to be thorough in considering not just every piece that can do the job, but every path your pieces can take to the needed line.