I've messed around a bit with these but I'm no expert. The way I did it was with vlc media player
http://www.videolan.org/vlc/ , paint.net
http://www.getpaint.net/ and unfreez
http://www.whitsoftdev.com/unfreez/ which are all trustworthy and free.
1.Using vlc media player: Go to View > Advanced Controls.
You will notice four new buttons, second from left is the snapshot button.You will use this to capture the screens.
2.Go to Tools > Preferences > Video.
3.Down the bottom make a folder/directory to save the Video snapshots in and while you're there set the format to jpg.
4.Watch the clip and get to the scene you want then click the snapshot button repeatedly while it plays until you're happy that you've covered the whole scene.
Now you need paint.net or ms paint to convert the snapshots from jpg to gif one at a time which can take a while, unless you have some kind of batch converter that I don't have. I never found a good converter for free.
5.Open your snapshots in paint.net and save them as gif's, one at a time into a new folder.
6.Open Unfreez.
7.Select all the gif's from the new folder in step 5 and drag them into the Unfreez box.
8.Set the Frame delay to your liking then click Make Animated Gif.
It takes some patience, I think I had a badly broken leg or something at the time

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Cropping can be done as well but I won't go into that. I'm sure there's much better and more efficient ways and means but that's how I did it.