Its not that hard. Learn a few very important but simple things and the rest you can DIY every bit as good as the pros.
One of the first things to learn is the frequency of the sound you want to control determines the wavelength which in turn determines the dimensions your acoustic treatment needs to have. That is why bass traps are great big tubes, while little 1" thick panels work fine for controlling midrange and treble.
That's where you want to start and that will be all I will now talk about.
Owens Corning acoustic panel is the raw material inside a lot of expensive panels. Its affordable, readily available at hardware stores or Amazon, and cuts and shapes easily with a knife or razor blade. Get a few 1" thick 2' x 3' panels. You might need 3 to 5 for a room but just get a few to start.
Back in your room, first thing you do, put one up against a wall. Just lean it up there. At head height if you can but whatever. Stand a few feet in front of it and talk. Move sideways. Talk and listen. Hear how it muffles? Do you want your room to sound like that? So you just learned first hand: use sparingly!
Now continue experimenting with the full size bare panels. Place one on each side wall where they will block the first reflection. Play some music. Move them around. Get a feel for what they're doing.
Do the same with a panel anywhere you think might be a reflection you want to control. Directly to your sides. On the wall between the speakers. Just keep moving the same few panels around. Don't cut or mount or anything. Just experiment.
This is where the vast majority fail. For some reason they all think someone else can tell them what to do. As if. I can tell you how. No one can tell you what.
Now you have a better idea, next testing, cut into smaller pieces. Probably you want to cancel first reflections. For that you only need a panel about a foot square, two feet tops. Panel this small is so light you can stick it on the wall with Scotch tape, or little push pins. Not tacks. Push pins. Smaller, almost invisible hole. Can tell I've done this?
Picture of my room, long ago, sort of in the middle of going through the process I'm describing here. https://www.theanalogdept.com/c_miller.htm
This was way too much. All the uncovered panels are gone now. The corner tune panels though, those you can see are done and covered with fabric.
That's what you do next. First triangles to go in those corners. Then long rectangles to do the longer ones. Just like the picture. These are fantastic and accomplish a lot without damping the room too much.
Last thing- when you are ready to cover with fabric. The sound you heard in testing is without fabric. In order to have that same sound your fabric has to be fairly open weave, like speaker grill fabric. Otherwise the high frequencies will bounce off instead of being absorbed like you want. It'll still work. Its just the tone balance shifts. Letting you know because it surprised me when it happened. Even though I should have seen it coming.... anyway fabric obviously is one where you really balance how it looks with how it sounds.
I don't even want to think about how many thousands it would have cost to have a pro do this for me. Instead of a hundred bucks or so.
Still, an awful lot of guys rather pay through the nose than do a little honest work. As you will no doubt soon see.....