The cut out in your motherboard tray may be large enough that you dont have to remove the motherboard.
Just clean the processor good.
First apply paste to the bottom of the cooler to fill the groves on each side ofthe heatpipes . just enough to fill the groves . Use the edge of a credit card or stiff business card to leave a flat surface and remove excess.Apply about a large grain of rice paste to the center of cpu.
place cooler on cpu and rotate it slightly clockwise ,counterclock wise then mount it with the screws.