How to fix this window on a stud wall?
These may be "insert" window replacements/upgrades.  An existing window has the jambs and parting bead removed.  Then this unit is inserted and the original stop bead on the outside and inside holds this unit in place.

You can use the same idea.. trim the rough opening out so this window fits snugly.  (Trim it like you would a door, with jambs and trim inside and, as needed, outside).  Add stop bead on both sides of the window to cover the gap and hold it in place.