How do I put all of the music in my iTunes library (12.2) onto my iPhone 6?
I am assuming that you have your 'entire Music Library' downloaded on your Mac. This seems like a simple case. 


Plug your iPhone into your Mac.
Open iTunes
Tap the iPhone icon
Under the "Settings" section header, click "Music"
Click the checkboxes "Sync Music" and "Sync Entire music library"