There are two ways you should be able to control this.
If you're syncing through your computer, you'd need to click on "Devices" and then "Manually select playlists to sync" in the top of the screen. You should then be able to select which tracks you are syncing on you device.
However, you can also sync directly on your iPhone within the Spotify app. Enter the playlists on your phone, and then select "Available Offline" at the top. That will sync that playlist to offline mode. By sure your phone is connected to a Wi-Fi network - it may take up a lot of data if you're using a 3G connection.