Photo realistic compositing takes a little time and is a real skill, Iam not sure that I am all that clever at it but here is the way I go about it.
I recently purchased a years subscription to Photoshop CC and am really pleased I did as I have been using Photoshop in various forms forever and a day! Suprisingly affordable when compared to buying the disc!!
While this walk-thru is PS based all the tools and methods do crossover to the main bunch of editing suites although you might have to 'play' a bit (Gimp, for instance insists on doing things the hard way adding things like 'Alph Channels for some reason!) But then Playing is just the best way to learn :-)

Click on the images below and view the images ful screen. Have Fun