I am surprised you say you don't have a clue where to start. I does help if you read threads as indicated. But to save you the trouble of doing that:
You force air into your sinuses exactly the same way as you force it into your eustachian tubes when trying to change the pressure on the inside of your eardrum. In fact you cannot do one without the other.
You pinch your nose, keep you mouth shut and blow hard until you feel the appropriate event happening, like ears popping or air flowing into somewhere (your sinuses) inside your head.
This is all about the simple physics of balancing pressures inside cavities in your head to equal the pressures outside those same cavities which necessarily changes as you change height in the air - or depth in water if diving.