Use sharp scissors like these – http://amzn.to/1sy2Sqk
Or just buy a device like this for $3.85 – http://amzn.to/1OfJgBm
Print the template from here – http://bit.ly/TMaxwM
This video teaches viewers how to cut a SIM card from a standard (Mini SIM) format down to either a Micro SIM (For the NExus 4 etc.) or Nano SIM (for the new iPhone 5). You can also use a pre-configured SIM cutter like the one linked above if you're uncomfortable cutting your own SIM card using scissors. Thumbs up if you found this video to be helpful & good luck cutting your SIM!
*Disclaimer – cut your SIM at your own risk!