First of all, you need to take a printout, describing the actual size of the Nano Sim. You can take measurements manually, but it is always better to print an original-size diagram of the Nano Sim. Place your standard Sim on the page, and cut the extra part.
If you don’t have printer, you may borrow the Nano Sim from your friend, and trace its image on paper.
When cutting your Sim after placing on the page, you need to hold it firmly. The best option is to stick the Sim onto the paper with tape, so that it does not move.
To mark the size accurately, use a ruler and marker. Highlight the edges with the help of the marker. Once you have marked it, remove the tape.
Now, it is time to use the cutter. You need to be careful, because you might end up damaging the Sim card. Hold the Sim firmly and slowly cut it.
To avoid any damages, you need to smooth out the edges. For this, you can use sandpaper. You have to be very careful again, because any extra force could damage the whole Sim. Once the edges are smooth, your Nano Sim is ready to use.