How to prepare (what all sizes and resolutions) mockups and UI graphics for an Android app in PS

I've seen similar questions asked here but they did not answer my question 100%. So please bear with me.

Now I am starting an app design project for iOS and Android. I've been designing for iOS for some time now but Android is totally a new thing for me. Android has so many devices at so many different resolutions and sizes that it is actually quite confusing to me at this point as to where to begin.

Is it possible to create your elements for the highest resolution and then use some method to scale them down for other smaller sizes? Or do we HAVE to do it manually and create them for each of the different sizes? What are the best practises and ideal flow for any such multi-platform requirement. Is there a way to easily reuse the iOS graphic assets? TIA :)