Lets try this with ImgBurn. It is the updated burning engine you find in DVDDecrypter.
Go to www.imgburn.com and download the program. It is free to use.
Look through the guides here
I can't emphasize this enough. You will learn more from the guides than I can explain in this post.
I'm going to make the assumption that you have already got your dvd video on the hard drive in the form of a Video_TS folder and a Audio_TS folder. If this is the case, AND the video is small enough to already fit onto a single layer dvdr, then you can use the Build mode in ImgBurn, make your ISO and burn it to the disk in one step.
If it isn't small enough to fit onto a single layer disk, then you'll have to reduce its size with something like DVDShrink. When you use Shrink, make the output an ISO. You can use the Write mode in ImgBurn to put this onto a single layer disk.
The other option with a large dvd video is to burn to a dual layer disk without compressing it with DVDShrink. ImgBurn can do this also, but you should use only Verbatim +R DL media.
This really isn't all that hard to do, and the guides will help a lot.
You should also look into a program called RipIt4Me. It can decrypt a commercial dvd, rip it to the hard drive, reduce it to the necessary size and then output to an ISO that you can burn either with DVDDecrypter (which is part of the program) or ImgBurn. It is free, very simple to use, and a good starting point for beginners in backing up dvds.