Doors and Windows - cultured Marble

Hi all, I just bought a small window air conditioner for my sons room. I have a 3 ton Carrier/Bryant heat pump and the upstairs doesn't get as cool as downstairs where the thermostat is located. His room also gets full sun throughout the day so My Wife and I thought this would be a simple solution. We purchased a small unit from Wallmart and now it's time to install. I put the unit together and was given two L brackets to screw into the unit and window sill. The sill is Cultured Marble. What would be the correct method to attach to sill. Thanks in advance.:)

With the condenser unit out of the frame, drill holes in the side of the casing at least 1" above the bottom, and screw it in sideways. The 1" elevation keeps accumulated water from leaking out before it goes out the weep hole in the back.