Zoom in to view the bottom left hand corner. Choose the lighter yellow (second line of colors) with the pen tool and cut the corner.
Move to the upper right hand corner and repeat the process.
Click on the paint can tool and flood the top part of the frame.
Next we’ll make the background a unique color (to be made transparent later). Un-zoom to regular view, choose a color (not yellow, white or black) and use the paint can to saturate the entire background.