Click on the FX in the clip and select Time Remapping - Speed
Hold down ctrl (cmd) to add keyframe on the clips white line in the timeline.
I find it easier to adjust the keyframes on the timeline.
Maybe the help is a bit clearer on this:
Ctrl (cmd) + alt (Option) to freeze
Ctrl (cmd) + drag to reverse
Thanks Ann. I've got it now, with the bonus shortcuts to freeze and reverse!
Thanks for this answer! A big help to me tonight