This process can be different on Macs and PCs.  Mac: iTunes → Preferences  PC: Edit → Preferences  " Click "Ok" once you've double-check that it is indeed selected. With this option checked, iTunes automatically makes copies of files you add to your library and puts them into the iTunes Media folder. If you wish to save space on your hard drive, you may want to delete the original files after you've confirmed the copies exist in the Media folder.  " Click "Ok." Copies of your media files with now be added to the iTunes Media folder. You've finished consolidating.

TL;DR in Chinese:
启动 iTunes 并打开“偏好设置”。 进入偏好设置后，点击最右边的“高级”选项卡。 点击“添加到资料库时将文件复制到 iTunes Media 文件夹”旁的复选框。 点击“文件 → 资料库 → 整理资料库”。 选中“统一文件”复选框。