The architrave is the frame on the wall around the window. Install the curtain rod holders on the architrave by marking out where the rod holders will go and predrilling screw holes. Screw the holders in place making sure that they're horizontally level with one another. If your architrave is level you can do this simply by mounting them flush with the top of the architrave. If it's not, use a spirit level to draw a line across the architrave and mount flush with the line.