Hi Jay,

To achieve this minimalist look,
a simple drywall return is often utilized. Meaning, narrow strips of drywall
are cut and attached around the rough opening. Then corner beading is applied
and the whole thing is completed with the drywall/finishing/painting
process. The key to achieving a professional look is to shim the drywall
returns to align with the window and not the rough opening. In other
words, the goal is to have an even reveal of the window frame all around.
In some situations, the desire is to cover most of the frame. For these
cases, a double or even triple layer of drywall return is utilized.

Drywall is typically the least
cost option to achieve this look. However, drywall does not hold up well
where condensation is likely to occur, especially at the sills. So, many
contractors will use primed wood in place of the drywall for the returns.
This will cost a little more but holds up better and provides something solid
to attach window coverings, etc.

Let us know if you have any other questions!