GiffGaff SIM, I used the faint golden rectangle that was about the size of a SIM chip to work out how far down it was safe to cut, but on the Vodafone SIM I hazarded a guess that the bracket-like lines were my primary clue. I ran my scissors along the outermost points of the brackets, being careful not to go over the lines (using my GCSE geography colouring-in skills). When I later cracked the SIM open, I discovered that my guesswork was spot-on. When it comes to cutting vertically, take note of the squiggly pattern in the middle and use that as a guide -- don't trim any further in than that. I first trimmed to the very edge of the metal, before using the two lines outside the central squiggly pattern as guides and snipped vertically, making sure I didn't cross the lines. The metal area on the GiffGaff SIM I used was already the right width, so fortunately I could cut (still oh-so-carefully) up the plastic sides and leave the middle bit be. One final thing is to remember which end of the SIM card was originally lacking a right-angled corner, as the final step in creating your nano-SIM is to make a tiny diagonal snip to fit your SIM into the tray. The moment of truth If your SIM fits and if you've looked through the leftover detritus and not discovered anything that looks alarmingly like an electronic chip, it's time to test your new nano-SIM