Reflection tutorial

Ok, so this is my first tutorial! Hope I managed to write it in a decent enough english. I decided to make a tuturial on reflection because I often see people doing the same mistake and using shortcuts that sometimes give a very inaccurate representation of what a reflection should look like.

That mistake I am referring to is duplicating the subject and simply flipping it upside down. I see that constantly! How many times have I seen an advertismement for a product using that cheap effect? They always do that to show you a flashy cellphone or whatever on a white background, with a reflection on the unexistent floor, know what I mean? Like THIS:

[link]

Anyway, I don't mean this to be pretentious or anything, I genuinely hope that the tutorial will be helpful! I'm not perfect either, and there's always place for improvement. The idea's not to do 100% accurate representations of reality, it's just trying to do something that "looks" right and that makes sense!

Thanks!