Like other maneuvers, descents have three key elements: planning, executing, and cross-checking to confirm the event. Your descent needs some careful advance preparation that will become easier with experience.
I frequently fly my 1975 Beech 55 Baron between Santa Barbara and Monterey, California, two coastal airports separated by mountainous terrain. I have several options of routing and descent patterns, but I am always thinking ahead as to what I will need and how I will accomplish my "nice to the ears and engines" slow-rate return to Mother Earth.