To burn a project to a DVD+R DL disc:
With the finished project open in iDVD, choose Project > Project Info and then choose Double Layer (DL) from the DVD Type pop-up menu.
Select the encoder setting you want to use. If you have more than 120 minutes of content, select High Quality.
Click the Burn button (shown below).
When prompted, insert a DVD+R DL disc into the drive.
You can use your internal SuperDrive or an external third-party drive.
If the disc isn’t blank, iDVD asks whether you want to erase the disc.