Replacing (key) sky in shot

[karma_koan]

Shot no. 1: people on bench with NY skyline in the back.
Shot no. 2: starry sky (will be shot far from NYC to avoid light pollution).

I want to key in shot no.2's sky into shot no. 1. What would be the best way to do this and what should I keep in mind while shooting in order to make the blending as seamless as possible?

I will be shooting with a Sony EX3.

Thanks.

[NoahKadner]

Depends on the shot... show an example.

Noah

[karma_koan]

I don't know how I can provide an example (sorry, this is in the very early planning stages), but this is kinda what I need to do:
http://www.lafcpug.org/tutorials/bas...tes_masks.html

I basically need a shot with the new york skyline in the back with a star filled sky. You can't really see any stars in nyc at night, which is why I wanted to shoot the starry sky somewhere else and key it into the first shot.

I also want to make sure that I avoid matching problems (although I will probably correct the final shot/fix any problems in Color).

Thank you for the quick reply and for your help.