1st you need to determine the size of the original. If the original is a DVD5 (under 4.37GB) then you could proceed to step 2 then a straight burn. If it is DVD9 (larger than 4.37) you would need to follow step 2 and 3.
2st you would need an unencryted source. A typical retail movie has protection so you would need to Unencrypt it, say with a certain DVD Decryptor
type program which is free. This would transfer the files onto your HD anless you used a program that alows you to read Any DVD
bypassing the protection so you may read directly from the DVD.
3nd The easiest method from here would be to run it through a program that compresses the actual DVD to bring it down from DVD9 to a DVD5 (recordable DVD) size. Most are near the same quality and speed (CloneDVD, DVDShrink, etc.) with the exception of one. Pinnacle InstantCopy definately takes longer but also produces a better quality. DVDShrink is worth looking into as it is a free program. These programs will burn straight to DVD+R for you allthough you may want to write to an image file 1st and use your own burning technique.
This is just a quick overview allthough it is really not much more difficult than that. Good Luck.