Short answer: dock the crank and touch nothing to descend.
When you set the fire on, the balloon will rise up. When you set the fire off, it’s acceleration will decrease and slowly start to descend.
There is a fire level that decides the acceleration. When you press A, the fire level is set to 100%.
Using the crank, 100% is the top of the console and it goes gradually to 0% at the bottom of the console. I don’t mention the crank gameplay because I find it too hard to play. Better dock the crank and only use the A button.
If the balloon takes too much time to descend it usually means that you pressed A for too long. Try to do short press instead. Still, long press are useful to go over the mountains.