Honestly, when I buy at my LFS I ask them to cut it for me before I take it home. The last ones I've cut off from shipping have NOT been great experiences. When I'm able to just jam the plug part down into open areas and hope for the best, I do.
+1 Bone cutters. You start the snip about a 1/4 of the way across the stem. Point away from anything you don't want broken or hurt. Squeeze with tips of cutters while holding disk section in hand.
I use wire snips. Just barely opens up wide enough for a plug. Like others have said, when it cuts things go flying. I hold mine under a paper towel or rag so when it pops I can contain the base and frag.
+1 to wire snips, thats what I use. Hold the flat round part where the frag actually is, point away from you and any glass, and the bottom will go flying when you snip.
I use a pair of channel lock pliers adjusted to just smaller than the diameter of the plug stem. Holding the plug in my left hand (to protect the frag, my thumb and fingertips go around the base "cradling" the frag in the palm of my hand), I work my way up the stem crunching a bit at a time. Do this over a shoebox size container to catch the pieces of the plug that break off.
ORA's plastic frag plugs either need to have the frag "popped" out (using the channel locks around the base of the plug, squeeze gently to loosen the frag from the base) or cut off the stem with a saw (a bit more of a pain).
Hope this helps.
Cheers,
Ray