I take the sheaths out of the holders, load the film, then pop them back in. I can get them in by putting in the bottom first, pushing down and popping it into the holder - I am holding the sheaths across the top two corners between thumb and forefinger.
To get the shealth out without touching the film, hold the holder vertically with the open part of the sheath uppermost. Angle the holder slightly such that the sheath/film would fall out (were it not clipped in) and give the holder a sharp (not too hard!) rap on its bottom edge and the top part of the sheath should pop out. (I hope that made sense, but practice in the light)
I actually find these easier to load than modern ones. I suppose it is because these were the first LF holders I loaded.