how do i burn a DVD disc from video i have downloaded to iMovie from an SD card??

You need DVD burning/authoring software.

You can use Apple's iDVD or Roxio Toast.

The full version of iDVD comes with iLife '09 and can still be purchased though Amazon or eBay.

http://www.amazon.com/Apple-MB966Z-A-iLife-VERSION/dp/B0014X5XEK/ref=sr_1_1?ie=U TF8&qid=1434833068&sr=8-1&keywords=iLife…

Roxio Toast Titanium needs to be purchased here.

http://www.roxio.com/

OR you could try the free, open source DVD burning software

Burn for OS X

http://burn-osx.sourceforge.net/Pages/English/home.html/