I use 2 apps.
1) To convert .avi or other video format to DVD (IFO, VOB) format, I use DeVeDe:
It generates the correct dvd tree and writes it on an ISO image which can be burned on a DVD media with k3b, for instance.
2) However, DeVeDe still is not a good option as an authoring system, mostly because it doesn't create a usable DVD menu. The solution I found was to use DeVeDe to convert the movies to mpeg-2 (use the option: "Only convert film files to compliant MPEG files") and then use a proper authoring system to create the DVD tree, along with the menu. In this case, I'm using DVDStyler:
which can be downloaded from a Fedora repository (extras or other, don't recall which one).
Hope it helps...