You need to fill your ears with air continuously on the descent.

If you don't fill your ears quickly enough, the air space behind the ear drum will start to compress, which will feel like a stretching sensation in your ear drums, and finally pain.

To avoid any under-pressure in your ears, fill them first on the surface, then about once per second on the way down. If you feel you aren't filling them fast enough, hold the rope and slow down.

Usually, when you fill your ears, your sinuses fill too. But if you have congestion or a sinus infection, your sinus might be blocked and unable to fill. You will feel this as a fairly sudden sharp pain in your sinus.

If you feel this, stop immediately! Sinuses can rupture quickly. Either hold the rope and try to clear the block without descending further, or return to the surface.

If you have a high-volume or inflexible mask, and don't fill it with enough air while you descend, it can cause barotrauma to your eyes. This will show up as a bloodshot eyeball when you return to the surface.

To avoid having to do much filling of the mask, wear a flexible, low-volume mask designed for freediving like the masks from Fins and Foam.

As you descend, your diaphragm needs to relax up under your ribcage to allow the airspace in your lungs to compress.

You should feel this as a pressure in the center of your chest. It isn't bad for you and doesn't do you any harm. Relax and allow your diaphragm to rise.

If you tense your diaphgram or push it downwards, your lungs can become underpressurized, which will manifest as coughing or wheezing once you return to the surface; difficulty breathing; tiredness; or blood in your spit.

You should treat lung barotrauma (aka lung "squeeze") seriously and stop diving for the day, and work on your diaphragm's response to pressure so that it doesn't happen again.