IMHO, and based on experience 1. There is no "easy" way. 2. You will always struggle with audio lag that pretty much makes it unwatchable. Other video types are a little friendlier, but it was too much hassle to convert framrates, etc. to keep audio in sync. I abandoned the project and bought a portable DVD player instead.
If you want to play DVDs on your DS, there isn't any freeware that'll do it easily. Chimi is correct that DVDFab will do it, but you will have to pay a yearly fee for the privilege. It depends on how badly you want to watch DVDs on your DS.