Push a chair up to the window where you want to install the grommet curtain panels. You need to reach the wall almost at the height of the ceiling to get accurate measurements.
Open the package containing the grommet curtains and lay them out flat on the floor. Measure the total height of the curtains and jot it down. Repeat the process to measure the width of each panel as well.
Climb up onto the chair in the center of the window and ask another person to place the end of the tape measure 1 inch from the floor. Stretch the tape measure upward until you reach the same height as the grommet curtain. Mark the location with a pencil line.
Place the end of the tape measure on the pencil mark and measure out to the right the same width as one of the grommet curtain panels. Mark the side location where the curtain rod should end.
Measure from the center line out to the left the same width as the curtain panel. Mark the location with a pencil to identify the end of the rod on that side.