The main ingredient of this insructable is white paper glue (water souble).
Other items you need are a glass work surface, a razor blade for scraping the glass and a small container for mixing the glue.
Step 1: Prepare the Glue
The reason for thinning it with water is so that the glue gets absorbed properly by the currency note, this also makes the glue layer thin and almost unnoticeable in normal use.
Step 2: Apply Glue
To apply the glue I took about a drop of it on my finger tip and applied it across one edge of the torn note. Remember that the glue becomes almost transparent when it dries.
Step 3: Join the Pieces
Place one half of the note with the glue still wet onto the glass surface and dab it a little bit so that the note sticks to the glass. Next place the other half of the note next to the first half half and align the torn edges so there is no gap between them.
If required apply some more glue along the tear and let it dry completely. This will leave a very thin membrane of dried glue on the note and likely to tear. So now we apply some more glue over the dried first layer, the idea is to build up the dry glue skin till it is reasonably strong enough to be handled as you would normally handle the currency.
Use the razor blade to carefully scrape the note off of the glass. Flip the note and repeat the procedure. Essentially you will be left with dried glue skin / membrane that would be holding the currency together.
Step 4: All Finished
When finished it is normally not possible to tell that the note has been stuck with glue unless observed closely. This technique may also be used to repair other paper articles such as old books, treasure maps etc