How can I create a screen shot from a movie to be used in GfD?
Internally GfD has two associated programs for this purpose:
GFDDSView and ChapEditGFD. GFDDSView is started by a double click on the video in the source list. ChapEditGFD is started from the 'Video and Audio Files' dialog. GFDDSView may be 'replaced' by another player in the permanent default settings (Viewer for Video Preview / AVS Preview).

GFDDSView crashes when I click on 'screen shot'
Either use ChapEditGFD or set up another player (which can save screen shots) in the permanent default settings (Viewer for Video Preview / AVS Preview) like Media Player Classic

When/why use MuxMan or dvdauthor
I don't have a final answer... The general differences are described here:
MuxMan vs. dvdauthor

How can I 'navigate' within the project?
You can use the project explorer, or choose 'Go to' from the menu, or double click on a button and select 'Load menu: …' from the context menu (of cause this only works if the button points to a menu)

How can I add additional menus and buttons?
Activate the 'Advanced mode' in the general project settings

How can I delete a submenu?
Titlesetmenus are automatically deleted if the associated movie is deleted. Free added menus can only be deleted within the project explorer: Open the node 'Submenus' in the parent menu of the submenu to delete. Mark the submenu and press 'del' (or right click on the entry). Only the last added submenu of the correspondent parent menu can be deleted.

Intro doesn't work/ authoring process crashed
- Avoid special characters like (ä,ö,ü) within the filename
- The intro must be dvdauthor compatible (contain navigation packs, continuous correct timecode…) See also: Using Intro

Authoring process crashed with ERROR: [???] MUX STATUS: Frame data under-runs detected!
Sometimes mplex fails for 'unknown reasons' (or at least I don't understand why) with this error. You can try to multiplex the video + audio files with the ImagoMPEG-Muxer and use the resulting mpg file in GfD. Most times this helps. If the video is 100% DVD compliant you can also try to use MuxMan as 'authoring engine'.

The menus are not shown/appear black on the DVD
There seems to be some kind of 'minimum frames' for a still menu (see also permanent settings, last point). Some DVD players do not show the menu if it consists of one single video frame. PowerDVD has problems with 2 to ~ 8 frames. Some DVD recorders seem to need about 12 to 15 frames (one GOP probably). For still menus with background music it gets even worse (more unpredictable how the player reacts). This problem should be solved with the new (0.6.13-GfD-1) version of dvdauthor, but if it still occurs, I have these suggestions:
- For maximum compatibility using still menus without background audio: Use 15 frames
- For maximum compatibility using menus with sound: Create animated menus
For MuxMan the only possibility is using animated menus, as there is no possibility to 'influence' this parameter. But this problem has not yet occurred with MuxMan as authoring engine, therefore using MuxMan is also a good way to circumvent such problems.

Known Errors:
- Dvdauthor crashed (SCR moves backwards...) when subtitles are used with 'special colors' (i.e. for NTSC streams).
Solution: Use spumux standard colors (don't use an own palette, nor edit the colors) or MuxMan
- Dvdauthor creates 'stuttering' (video or audio) for still menus with sound if 'Full length' encoding was used
Solution: Create an animated menu or use MuxMan
- Dvdauthor creates 'stuttering' (video or audio) at cut positions if cuttermaran was used with frame accurate cutting
Solution: Use MuxMan if possible
- Video and audio are not in sync with MuxMan as authoring engine: No solution yet.
- Foreign characters are displayed wrong on menus even if they appear correct in the text edit window
(see: http://forum.videohelp.com) No solution yet.