Step 1. Assemble
Lay the window frame on a flat surface and then sit the window sash into the window frame rebate.
Make sure that the gap between the sash and window frame is even all the way around.
Screw two hinges to the top rail of the sash and the head of the window frame, one each end.
Use either butt hinges or T-hinges.

With the window still laying on a flat surface fix 70mm x 25mm (1x3) trim to the perimeter of the window frame.

Step 2. Install
Place the completed window into the window hole (rough opening) in the wall. Ensure that the window is level and plumb and then fix it in place by nailing through the trim into the wall.

From inside the room, nail the window frame side jambs to the wall rough opening frame.
The rough opening will be (or should be) slightly bigger than the window frame, so packing pieces will need to be placed between the jamb and the wall frame where nailing occurs.

Step 3. Inside finishing
Usually the window frame is of an exact width so that the inside of the window frame will be flush with the inside of the wall, allowing for a molding, architrave or casing to go neatly around the frame.
However......
in this case the window frame is wider than the wall and therefore projects in to the room.
Finish by fixing a 20mm x 20mm (3/4"x 3/4") square bead (piece of wood) around the perimeter of the window frame to the wall.
Fit hardware of choice. In this case a quadrant stay has been fitted which allows the window to be opened and locked at various positions.