Make a tube by cutting off a small section of each end of the envelope to form a tube. Make a paper doll, which can be a simple strip of paper with a person drawn on it. Make sure the doll can slide in through the slits you create in an envelope.
Flatten the tube and cut two slits into the tube. Just enough to be able to slide your doll through it. This will be the backside of your envelope and the secret to being able to cut your doll in half and restore her back.
Make sure that the slits are wide enough so the paper doll can slide through them.
You're ready to perform the trick.
You can put the doll in the envelope ahead of time or in front of your audience.
If you decide to slide the doll into the envelope in front of your audience practice
until you are comfortable doing so.
Slide the paper doll into the paper tube. Slip the end of the doll through the first slit and out of the back of the envelope. While you work the doll into the tube, you'll need to slip the doll back into the tube through the second slit.
When sliding your doll into the envelope it appears as if it is entering the envelope on one side and emerging from the other. The spectators don't know that the middle of the "person" is actually underneath the envelope and not touching it.
Everything will appear normal.
Now for the fun and illusion.
Cut the envelope and doll in half making sure that you are cutting under the envelope but above the paper doll.
After you magically cut the paper doll in half, hold the two pieces of the envelope together and pull out the paper doll. It will appear that the paper doll has survived the cutting. You can allow spectators to examine the doll, but quietly crumple up and discard the paper tube before they ask to see it.