The hardware store sells a small tool that fits INTO the two holes/notchs in the head of the Molly…….Cheap stamped soft metal…..Shaped like a Y….only about 2″ long….(comes free in a box of mollies)…..it allows you to hold the molly from spinning while you screw the anchor in place…..
In lieu of the special tool you could probably use a pair of needlenose or gas-pliers to hold the head/still until the molly starts expanding
“…measure once…..cut twice….throw that one away and cut a new one….”