Download AnimStack script and read the complete documentation here: http://registry.gimp.org/node/26501

How to install: click the above link, scroll to the bottom, download "animstack.scm". In GIMP, go to Edit - Preferences - Folders - Scripts, and put animstack.scm into the first folder listed there. Then go to Filters - Script-Fu - Refresh scripts.

Other recommended scripts:
Cycle layer visibility - http://registry.gimp.org/node/20080

Table of contents:

00:37 - Basics (add background and foreground to animation)
03:40 - Tag parameters (flashing text)
06:04 - Roll (more flashing text)
10:59 - Advanced usage (snow animation)
19:46 - Ranges (set different frame delay for 100 frame gif)
26:16 - Duplication (smooth transitions between frames)
30:58 - Exporting (how to export to GIF)

Extra AnimStack 0.3 tutorial: http://youtu.be/49FGgONSduM

See also:
My Ponygoons GIF tutorial: http://www.mlponies.com/forums/viewtopic.php?p=76091#p76091 (warning: ponies)

Music used:
No Trigger - New Brains (intro)
Titus Andronicus - Theme From "Cheers" (outro)
and a bunch of other tracks for chapter title cards

Shout out to AnimStack early adopters:

http://hellyeahgimp.tumblr.com/
http://gimpedia.tumblr.com/
http://www.gimpbnksb.ru/forum/29-1122-1