Hey alex, welcome to RO.

There are some general guidlines to acoustics, although trial and error will prove to be the ultimate answer. Im going to highlight the basics of a RFZ (reflection free zone) and urge you to experiment with both or all possible orientations.

Speaker Location:

First, best bass response is usually acheived when the speakers fire down the longest dimension. This places them on wall B. In this orientation youve got a window in a common studio location, where its good to look out of during editing or whenever, and acts as a bass trap. Youve also got the cupboard in the rear to act as a bass trap (when stuffed with insulation or similar), the entrance at the rear (door also acting as a bass trap), and youve got symetry from left to right, which you need for accurate panning.

To place the speakers, Start about 38% back in the room, mark a spot about 1.5 feet behind, and make an equilateral triangle from that point and tbe speakers. Common speaker locations are directly in front of the wall, or spaced off tbe wall 2-3 feet. Then trial and error from there.

Acoustic Treatment:

The mid/high absorbers could be rigid fiberglass panels, acoustic foam, and/or moving blankets. Youve already got curtains for the window. You may want to add more, making the front wall completely dead. Then you'll want a cloud of absorbers above the mix position ideally spanning most of the width of the room, and lenghtwise from above the speakers to a few feet behind your head. On the sides, and rear, youll want to sit at the mix position and have an assistant drag a mirror across the wall, anywhere you see a speaker in the mirror is a refection area, and should have absorption. The left and right walls should be symmetrical and have treatments in identical locations. For the rear, id hang a moving blanket (treated with dure retardent spray) on the cupboard, and stuff it with pink fluffly building insulation, with some plastic sheathing sealing in the insulation fibers. This turns the cupboard into a broadband absorber / bass trap.

For bass traps, youll want to install them in every single corner available. Youll want to search tbe web for super chunk bass traps. There are two types, one is a panel stradeled across the corner, with insulation behind. Or its little rigid fiberglass triangles stuffed into the corner.

Its worth noting any time you use fabrics make sure you it has a weave you can breathe easily thru them, so sound can go thru easily. And you want to make sure its treated with fire treatment spray or pre treated from the factory.

Mounting options can range from impaler brackets, to mic stands, to wooden frames.

Wether you diy, or go with pre built is your choice. Diy make sure you use common materials like rigid fiberglass, fluffy pink insulation, and burlap. When you use pre made, make sure its from a relaible company like ATS, primeacoustic, auralex, GIK, ect. If its not a mainstream company, request test data for the products, and make sure its from a relaible lab, and the response curve is similar to mainstream products if the same type.

Also you want to download tbe free REW room testing software, to help you get the best response in the room, and best speaker location.

This should be a good start. Once youve got as much bass trapping as possible, and your mid/high absorbers and cloud up, and your speakers in the best location, then you can look into tuned traps, and spot treatments for fine tuning your room.

Hope this helps, acoustics is a deep topic so feel free to ask questions.

Hi and welcome to RO Alex !
+1 on DIY bass traps. There is many youtube videos explaining how to build them.
What most do not realise is that bass traps also affect high frequencies. But if you go for foam it will only affect highfrequencies and mostly push your room off balance.
It's not that foam is bad but it should be use at the end if necessary.
The first must have absorbsion (where to place your first bass trap pannels) is on the first reflection point.
A nice trick is to make the speaker placement first (as suggested by K) and then move a mirror on the wall until you see the cone of the opposite side speaker (from your listening position). Put your pannel there !
Start to listen your favorite music and record live instrument in the room and figure out what the problems are. Then add treatment accordingly.

Having a treated room makes a lot of difference, good for you for working on that crucial aspect of recording !

Here is an exemple on how to find your first reflection point.. Please forget about the foam ;)