You will need:
2 cell phones (one to test with and one that causes interfernce)
1 9 inch piece of foil (tear off 9 inchs of foil)
a flat surface to fold foil.
Parental Guidance if needed.
Step 1: Folding the Foil
Place your phone on top of the foil, near the top edge.
Now fold the foil in the middle by bringing the bottom of the foil over the phone. Meet the two jagged edges together and crease the new bottom edge.
See diagram (fold on red dotted line)
Step 2: Making Side Folds
FOLD on first red outside red line. Then FOLD again. Repeat for other end.
Step 3: Finished.
A small wallet sized one may block RFID chips in credit cards/Drivers License
Although I am not condoning illegal activity a larger version can be made to fit a libraray book. It might be enough to block the scanners from reading the RFID tags affixed to library books incase you wanted to take home a reference book.