How can I copy a DVD using iDVD?

You can't. iDVD was a video DVD authoring (creating) app.

What type of DVD do you want to copy? A video DVD? If so is it a commercial one or one you created yourself? Or do you want to copy a data DVD disc?

First make a disk image of the video DVD disc with Disk Utility.

Then eject the disc and open the disk image to see the contents. There should be two folders:

Video_TS and Audio_TS. Launch DVD Player and play the video DVD from the disk image to verify it's a good copy.

Put a blank DVD disc in the option drive, Control (right) - click on the disk image and select "Burn to Disc" from the contextual menu:

First follow this workflow to help assure the best qualty video DVD:

Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process. 

To check the encoding mount the disk image, launch DVD Player and play it.  If it plays OK with DVD Player the encoding is good.

Burn at the slowest speed available (2x-4x) to assure the best burn quality.  Always use top quality media:  Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.