Is there a way to easily use music from Google Play Music as ringtones on my phone?
Download the song and use something like Audacity to edit it. Save to your ringtones directory on your phone. Nothing else out of the ordinary needs to be done.
I am using Music (latest up to today) and TTPOD as my MP3 players.
After Googling, I suppose I can set my MP3 ring tone, by long press on the song through Music application. However, once I long press, there are only the following selections
I was expecting I am able to get a selection choice Set as ringtone, but I didn't find one.
Set as ringtone
May I know what is the "correct" way to set MP3 as ringtone in Android?
This isn't the "correct" way, but IMO it's a good way: use Ringdroid. With it, you can cut a suitable part of the MP3 to be used as a ringtone. This way you can get rid of those pesky intros on songs that have a slow start and save space by not having the entire MP3 duplicate in your ringtones folder.
In last versions of Android (at least in Nexus S) it's kinda hard to set MP3 as a ringtone. There was an option before in the menus called "Set As" but in the new Music app it disappeared.
What you can do is copy your mp3 to /sdcard/media/audio/ringtones,
that ringtones folder might be empty or even not exist, in this case just create it.
Then you go to Settings -> Sound -> Phone ringtone and select your mp3 from the list.
Settings -> Sound -> Phone ringtone
I use Ringtone Maker, which is a free app. This app allows you to select a part of your song as the ringtone. It has some preselected portions (based on popular edits included in the database), or you can select your own portion of the song to set as the ringtone.
If you simply want the beginning of the song as your ringtone, @bakytn's answer is the way to go.
Some other music apps will give you the option to choose a song as a ringtone, such as the $5 PowerAmp, but I'm not sure what they offer in terms of choosing a portion of the song.
I found a clear and very good guide to set your own ringtones to the Google Nexus 5. I hope this helps.
Download MP3 music download app. Select song tab, and press the selected song. You will get option set as ....
set as ...