In addition to reading playlists transferred from iTunes, the iPhone can generate new playlists from music already on your device. The next time you sync your phone, these mobile playlists transfer back to your computer, so you can play them in iTunes as well. With iOS 8, you can add songs to playlists one at a time or in bulk by album, artist, genre or composer.
Open the "Music" app on your iPhone from the home screen. If you don't see the app on the current screen, swipe to either side to move between pages of apps.
Swipe down on the screen while viewing the Playlists tab to display the "New Playlists" button. Press it to start a new list.
Enter any name for your playlist and press "Save."
Switch to the "Songs" tab and press the "+" icon by every individual song you want in your playlist.
Open the "Album" or "Artist" tab, or tap "More" and pick "Composers" or "Genres" to add songs in bulk. You can also import another existing playlist. In whichever category you choose, pick an item and swipe down to reveal the Add All Songs "+" button. Press it to add the entire list of songs to the new playlist. When you finish adding songs, tap "Done" at the top of any screen.