The most effect for the least material is a triangle shape in the upper ceiling corners. 12" on a side is plenty.
Next most effective is vertically where the walls meet, and horizontally where the walls meet the ceiling. 12" wide by however long you want to treat.
The most common material used is Owens Corning 703
https://www.google.com/search?q=owens+corning+703&rlz=1C1GGRV_enUS765US765&oq=owens+corning+... It comes in different thicknesses, is available all over, and if you hunt around they even have a spec sheet with graphs showing acoustic absorption of the different panels by frequency. OC703 cuts easily with scissors or knife, or like butter with a table saw. Its light enough to be put up with little push pins that leave an almost invisible hole in the wall, making it trivial to experiment with locations.
Then when you settle on what and where its easily covered with whatever fabric you want. Just be aware your choice of fabric will affect performance- the more transparent like speaker grills the less change, the more tight and strong the weave the more it will reflect like any other surface.