First of all, grab the loose rope with your right hand. The said rope goes down to the cave bottom. No matter what, you should never leave this rope during descent as it is your brake hand.
If you want to decrease the descent speed, lower your right hand below your waist while holding the rope. The rope will be prevented from running through the ATC. Keep your hand a little low. At least, try keeping it at the same level if you cant keep it below your waist.
The speed will be increased if you raise your hand too much. It happens because the rope flows quickly through the ATC. You might have got a feeling that It can put you in some serious problems.
By now, you have understood that your right hand is your brake hand. Your left hand is required for guidance. The rope which is tied with anchors will be held by your left hand. Use your guide hand to put the rope through the ATC device. That is how you rappel.
Always keep your legs at a 90-degree angle to the cave wall. You can walk down slowly as you descend. Please take care of slippery surfaces while dropping as it may imbalance you.
Sometimes, there will be no cave walls. In that case, you can't keep your legs on any surface, and you rappel freely. You must watch out for obstacles to be safe, even if you are falling at a slower speed.
When you rappel freely without any wall support, you need extra friction. You will need specialized devices known as rappelling descenders. Many high-quality devices are available in the market.
At some locations, local guides may also provide some of the equipment. However, I prefer using my gear because I am aware of its function and limitations.