Summarize this article in one sentence.
This is a creative method for hiding a cut in the side, using Papier-mâché paste to seal the opening. If the papier-mâché ends up too thick, too weak, or too sticky, its presence will be obvious. This method is best used on letters that won't be carefully examined or heavily handled. You may also need plenty of time to adjust the papier-mâché seal. Hold it up to a bright light or window so you can see the shadow of the document inside. Take a mental note of its position and take care not to disturb the document inside. Using a pair of sharp, small scissors, snip off an absolutely tiny corner, preferably the bottom, while ensuring you do not snip the document. Cut along the crease of the side of the envelope, not removing any width but effectively opening your envelope. You can now read the document inside, or insert any materials you forgot to include in your mailing list. Mix white flour and water together to a fairly runny consistency. Test this on a spare piece of folded paper to see if it will stay stuck together once dried. Add more flour if necessary until the mixture dries strong in a thin layer. Boiling the flour-water paste will make the mixture dry clear instead of white or off-white, but will also make it weaker. Boiling is crucial for dark-colored envelopes to make the papier-mâché less obvious. Using a letter opener or some other smooth-edged instrument, spread the paper mâché paste to the edge of the cut in the envelope. Make sure not to get the document inside wet. Wait until fully dry. For a stronger seal, apply a second layer of papier-mâché paste. Repeat until there are no holes visible and the side stays stuck together. Work slowly to avoid scratching the envelope itself, especially if there is ink on the edge you're sanding. After the visible paste is removed, the envelope should look like an ordinary envelope that has never been opened.

Summary:
Know the risks. Hold the envelope in front of the light. Cut the corner of the envelope. Slice open the short side of the envelope. Make a small amount of papier-mâché paste. Seal the cut with Papier-mâché once finished. Wait for it to dry and repeat if necessary. Using fine sandpaper, sand away the rough bits of paste from your envelope.