Partially open both windows and paint the interior faces of the rear sash, the underside of the front sash and the inner, lower rebates of the window frame.
Reverse the position of the windows and paint the remaining interior faces of the rear sash, the top side of the front sash and the inner, upper rebates of the window frame.
Next paint the front face of the lower sash, using long brushstrokes along the grain of the wood.
Don’t overload your brush with too much paint as this will encourage runs.
Paint the face of the frame and cut in the edge next to the wall.
Finally paint the window sill, on the underside first, then the sides and the front face, finishing off with the top surface.