This is how I made those animgifs that I added to various spell articles. I'm not saying that this is the best way to create them. It is only my way of doing it. Maybe someone gets inspiration from it and comes up with a better workflow.
I'm a Windows user. I have no idea what kind of tools are available for other operting system users, but probably there is a broad selection of utilities for Linux or MacOS X users, too.
My weapons of choice are Instagiffer, IrfanView and UnFREEz. And, of course, a media player, for which I picked VLC. I can imagine that there are other utilities that provide all the necessary features, but these are what I know.
First thing to do is getting raw material. Unfortunately, the most often available fansubbed video files are less than optimal choice. They are most of the time fused with the captions, which would necessitate cleaning. That is often very hard to do, and impossible on my level of expertise. The raws or dubbed versions (any language will do) are the perfect chioce. My searches resulted in only one available raw source. It has the word garuda in the file names. (Man, it was like a blind dive into the unknown for me when I did the necessary registration to get those files.)
Second thing is finding the appropriate image sequence. Well, there is obviosly only one way to do this: watch the anime. The Wiki contains quite a lot of pointers which episode should be watched to look for the scene to have as illustration source. Once I found the scene of interest, I note the time of start.
Third step is extracting still images. I use Instagiffer for this in my workflow. For image extraction, I set original image size and maximum quality. I set the previously noted start time and change the sequence extraction time to cover all of the magic action. Based on how many frames the action will take, sometimes I change the framerate of extraction. The default value is 10. For short actions I may increase the rate because it will produce more fluid animation. However, if there will be a lot of frames, it is probably necessary to decrease framerate, otherwise the resulting animation will be too large. For the illustrations I made, I used values 8, 10, 12 and 15, IIRC. If the image sequence contains superfluous leading and trailing frames, it is possible to exclude them during exporting the images.
Fourth step is image selection. The sequence often will contain images that are not necessary to illustrate the spell. E.g. additional views from other cameras, sequences repeating for different subjects, or sequences intended to emphasize the effect. Viewing the image sequence in IrfanView by displaying the images one after another, and decide which ones will be thrown away. It is a good idea to actually remove these from the working folder, so they can't crawl back into the final animation.
Fifth thing to do is resizing and transcoding the images. I use the batch processing feature of IrfanView for this. I select 300 by 168 pixels for target size with retaining image aspect ratio. I select the maximum output quality for the gifs. When the sequence has captions, and the captions are a single line in the bottom of the images it may be possible to crop them without losing to much details on the action. When cropping the images, the aspect ratio must be kept the same. E.g. in a 1280 by 720 source I crop the lower 1/9th of the images (630 pixel high from top), and 1/16th of the image from both sides (origin 80 from left, width 1120). 1280/720 = 1120/630 = 1.777777777777778
Sixth step is creating the gif file. You simply drag and drop the transformed gif files into the UnFREEz utility. One thing to decide is the frame delay. The two factors influencing the value is how fast animation would best illustrate the action and what was the frame rate of extraction. Lower origninal framerate requires higher delay value, higher original framerate dictates lower delay value. Similarly, slower gif animation speed requires higher delay value, swifter gif animation requires lower delay value. IIRC, I used delays from 4 to 7. I alwasy turn on looping. The tool will generate the animated gif at the click of a button. The image naming conventions of the Wiki must be complied with.