Please note: This is not nearly as complicated as the directions make it seem. Just start to do it and you'll get it.
You will need a long business envelope and a pair of scissors. Seal the envelope and draw a stamp about 1/2 inch from the right edge of the envelope where a stamp would go, draw some cancellation lines. Draw squiggles where the address would go and the return address in the upper left corner. You will be making four cuts, three of them starting from the bottom edge of the envelope. The envelope is in front of you, as if you were reading it. The first cut is 1/8th from the right edge, cut the strip all the way off, to open the envelope. The second cut is about an inch or so from the right edge, cut most of the way, but not cutting of the stamp. The third cut is on the left side edge, leave about an inch margin and not cutting all the way. The fourth cut is cutting open the bottom edge of the envelope between the two vertical cuts. Or if it easier to understand, just cut the fold, but only between cuts two & three, the vertical cuts.
So here is the dialogue as you demonstrate this to the people you're telling the joke to:
Sr. Mary Margaret got a letter at the convent, and she did like a lot of people do, cut off the end of the envelope. (Make cut #1, cut off 1/8" of the entire right side of the envelope).
She couldn't get the letter out (make motion like you can't get the imaginary paper out) so she thought if she cut a bit more, she could get a better grip on the letter, but she didn't want to cut off the stamp, since she saved stamps so she stopped (make cut #2, cutting to the left, about 1 inch. stopping before you cut all the way).
Then she thought she could get at the letter from the other end (Make cut #3 about an inch from the left side but stop before you cut all the way, leave about 1 1/2 inches, like you did for the side with the stamp on it) but she didn't want to cut off the return address.
Now she is really wondering who the letter is from but she still hasn't gotten the letter out of the envelope so she decides to cut the bottom edge (Make cut #4, the bottom BETWEEN the two vertical cuts)
Hold the envelope by the two large "flaps" that you have just made by cutting the bottom edge (so the left edge of the envelope is now towards the table and the stamp end is towards your ceiling. (Open it up like you were opening a book to read) and you should see why Sr. Mary Margaret knows the letter came from Father Smith. And say aloud " Father Smith.!"
If I get a lot of queries that people cannot figure out how to do this, I may have to give it away but I want people to get the chuckle out of just doing the joke without giving it away here. Let me know if you figure this out.