1. Connect phone and computer
2. Select option
Transfer files from your computer to your phone, go to 2a.
Transfer files from your phone to your computer, go to 2b.
3. 2a - Transfer files from your computer to your phone
Click Add File to Library....
Click Add Folder to Library....
Depending on your settings in iTunes, the transfer may start automatically.
Follow the instructions on the screen to select the required settings.