Let whatever is in your nose and sinuses drain slowly out of your nostrils, while the hot water of the shower falls on your chest. Put a finger across your top lip
if you don’t like the salty water getting in your mouth.
After about 20 seconds, snort inward, so the mucus goes into your throat, then do some hawking, and spit it out of your mouth. Repeat once or twice. Then, close one nostril, and blow out the open nostril as slowly and gently as you can.