Ring tone from own music

[Twoormorecharac]

Hi, I'm new, apologies if this topic has been discussed. If so please just lead me to the right thread: 

I want to personalize my ring tones with music that I downloaded on my phone from Google Play Store. Read somewhere that Ringdroid should do the job, got the app on my phone -  but can't figure out how to do it as my albums/songs are not automatically on there. I just find everything that had been pre - set on the phone. Please explain to me what to do - but like you would explain to a five year old as I'm a smartphone idiot... 😥 

So - each single step please! 

Thanks, Harry