How to convert a video file into an Animated GIF in Photoshop?

I was trying to convert an MP4 into an animated GIF for an answer on here.

I tried opening the file directly in Photoshop, and then clicking on the timeline menu and choosing both:

• 
  

  Convert Frames > Flatten Frames into Clips and

  
  


• 
  

  Convert Frames > Convert to Frame Animation

  
  

but neither of them worked. They appeared to just flatten it into a single image.

How can I create an animated GIF from a video file in Adobe Photoshop?