I curve the film a little, so it slides inbetween the sides of the reel,
fix it into the center post (some reels have a push tab of sorts that I press with the index finger of the hand not holding film, keeping in mind to only bend the tab very little; Nikor and Hewes reels have the little pins to to which you anchor against sprocket holes on the film, Nikor has 3 pins and Hewes has 2, but I found Hewes easier to use),
then while still keeping that curve of the film, start rotating the reel while it's standing on the table. So the opposite of what others mentioned, but works well for me as long as I keep the curve in the film.
As soon as I figured that out with a dummy roll of film, loading in the darkness got much easier.