I’m replacing the siding and windows on a property, and this has given me an opportunity. The garage is attached, and it is a fairly simple process to add a window to it. See the photos below from the step-by-step process.

Removing some oriented strand board to reveal the studs.
Wall removed exposing the studs and the contents of the garage.
Removing the existing studs to make room for the window frame.
Measuring the opening.
Repositioning the original studs to accommodate the new window frame.
Securing the studs into position using wood screws. I also used nails.
Creating a header by making a sandwich of two, 2 x 8s and a piece of 1/2 inch plywood. The header needs to be the thickness of the 2 x 4 stud.
The completed window frame. The supportive stud is called the King Stud, and the inner stud is called the Jack Stud. Also note the header.