How to properly add admob ads to a (unity) android project

hey guys, so I'm not exactly sure if this is the right place to ask this question but oh well... i hope that maybe someone can help me out. So the thing I've been trying to add admob adds to my unity android game for quite a while now and it seems that nothing works for me.I have tried numerous guides and tutorials and A LOT of different plugins however nothing works. I mean i don't get any errors, i am able to install the game on to my device and it works and all....but ads don't show no matter what. for example i followed this tutorial https://www.youtube.com/watch?v=NR1xOMxLm04 but when i run my game i still don't see any ads.The only times when I had any progress was when I tried importing my project to eclipse and then making some changes in the manifest.xml and the main java script (as seen in https://www.youtube.com/watch?v=FiFGmNcsm7Y). But this way the ads are displayed the entire game and i want them to be visible only in some scenes. So guys please, If anyone has any experience in adding admob ads to a unity project PLEASE tell me what you did to make it work.(or maybe tell me what could be the problem).I'm pretty new to programming and I finally finished my first game. I never imagined it would be this troublesome to add ads. (P.S i'm trying to add banner ads).