The only step by step I remember was shown by beav of the HAsbro designer and the Youtube video link doens't work anymore.
There are a bunch of different methods out there though. So finding one that works for you isn't that hard.
Hasbro's method. Richard, the designer who showed us how to do it uses either a wood or plastic block shaped like this.
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Hey sets the point of the block under the armpit of the figure and whacks the figure witha rubber mallet. He was able to do this in one whack and it took about 1 second.
The other method involves using an adjustable wrench and either sockets or hex alan wrench bits. I personally found the sockets frustrating try to keep the figure where it should be and prefer Chiefs method as shown here.
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Matthew wrote:
I use a razor blade. Start under the arm and work your way up. Do this on both sides. Then I slide a small flat head screwdriver (or any metal thing dull and flat) then pry it open. I've done this for 3 torsos without hurting any of them.
Others have found success simply using an x-acto knife to cut the torsos in half. This was very tedious for me and left the torso with a noticeable gap.
My method is sort of a Matthew/Hasbro hybrid. I score the seem with an x-acto and then jam a small flat screw driver into the arm pit and whack the screwdriver with a small hammer. This has been 100% successful for me on a dozen torsos although I have had to take a couple of whacks on Scarlett

because of the skinny figure. No broken torsos and no missing digets = win for me.
Hope all of this helps you.