If playlists, artists, albums, and genres just don’t work for your music management and sync style, iTunes lets you manually add individual songs to your iPhone, iPad, or iPod touch. As long as your device is plugged into your computer or you’ve enabled the new WiFi sync feature, you can drag songs from your music library onto your device in the iTunes sidebar.
iTunes will keep track of all manually added songs at the very bottom of your device’s Music info tab, pictured here. This is also where you can remove those songs.
Note that iTunes has long allowed manually adding other media like movies, TV shows, and podcasts from their respective device info tabs.