Sound and acoustics are pretty simple. It all stems from the fact sound radiates in all directions as waves. The lower the frequency the longer the wave. At the high end the waves are only around an inch. So anything flat and larger than an inch will reflect these like a mirror. Bass waves are at the other extreme, 50 feet long. In other words all rooms are small compared to low bass. This is why all rooms have low bass problems.
In practical terms then what you do is think and try and visualize the sound waves. They come off your speaker radiating out in all directions not just towards you. If you sit wherever you are going to sit and imagine mirrors on all the walls, wherever you see a reflection of the speaker that is where sound will go and bounce and get to you. Which is fine. Except you don't want it to be a mirror, you want it to break that reflection up. This is called diffusion. Or you can try and absorb it, this is an absorption panel.
Too many diffusers and the sounds break up real nice but bounce around forever, the decay is too long, and this muddies clarity. Too many absorbers and the room sounds dead like being in a clothes closet. You want a nice balance. Most rooms get there naturally just by putting carpet and furniture. Audiophiles however can really screw it up big time. Pay attention in a good room where and how much is carpet, fabric, hard flat surfaces, diffusion, etc. You want that balance.
The ceiling is the same as any other large flat surface. Either break it up with diffusion or absorb, either way is better than nothing.
This can be done creatively. Doesn't have to look like a science project. Any architectural feature that adds angles will diffuse, anything with fabric will absorb. The size of the surface determines the frequency (because remember wavelength) while the thickness of the absorber tunes the frequency there, again because of wavelength. Lower frequencies, longer wavelengths, larger thicker panels.
People will sell you on all kinds of devices for doing this. Do you have two hands? Do you have two ears? Then clap your hands and listen. Walk somewhere else in the room and repeat. I have done this for people many times, always they are amazed how easy it is to hear these things. They just never tried.