1. Do not lean against any part of the airplane as the airplane is vibrating and that will ensure blurred pictures. Do not lean the lens against the window.
2. Set the camera to aperture priority and set the aperture to f11. Since everything you taking a picture of will be at infinity, f11 will give you the best sharpness and let the camera set the appropriate exposure time. Make sure you use a exposure time of at least 1/250 (Bump ISO if necessary)
3. Use the middle part of the airplane window (least distortion) if you're not able to have the window / door removed (private plane only of course)and don0t shoot at an oblique angle (incredible distortion)
4. Choose an airplane window that is least scratched
5. Experiment with a polarizer filter to reduce reflection even further.

Cheers and good luck
Peter

I'd recommend shooting at shutter priority so that the camera can be set to counter any movement or vibration. Corresponding aperture isn't an issue as long as you're focused on infinity. Everything on the ground will be out of the range of focusing. Most cameras allow setting on infinity and I would use that over any auto focus setting.

Sharpest aperture is not necessarily F-11. It's usually two stops down from wide open. but needs to be tested on a lens by lens basis.

One thing which has not been mentioned that in most middle of the day conditions which limits aerial photography is low contrast caused in part by atmospheric haze, and is added to if shooting through aircraft windows.

You can overcome this low contrast by using the cameras option menu to increase contrast and possibly saturation.

What ISO do you think I should use?

It depends on how fast the lens is and what aperture you'll be using. I always try shooting at the lowest ISO for the highest quality image, and then compensate according to the amount of light, camera movement and subject movement. The answer to your question will come from thinking on location.