Glad you asked
1. Find a video you like and scan through it, picking your favourite parts. I try to keep them to around 8-12 second clips
2. I screen record from my laptop and trim the clip to the desired length
3. ezgif.com is a great place to convert it to gif
4. Optimize the gif. Compress as necessary to get it to under 20 MiB or so, whatever the limit is on FCN or your image host site
5. Save that file to a folder and post away