Never try to cut your regular/micro sim card to get nano sim card. Metal part dimension of regular/micro sim card may not be same for nano sim card. You can easily replace your existing Airtel/ Idea/ Vodafone regular/micro sim card to nano sim card for nominal charge ( something around Rs 100) or free in some cases. It is always better to replace your regular/micro sim card to nano sim card from Airte/ Idea/ Vodafone sim card seller or costumer care center. Once you get nano sim card, you not only can use it in the device which support nano sim, but also you can use it in those mobile which require regular size sim or micro sim by using nano to regular sim adapter or nano to micro sim adapter easily. The cost of adapter is nominal.
To get the nano sim, you are required to visit to nearest sim card dealer (seller). Ask them to replace your existing sim card to Nano sim card. They may ask you some document like address proof, Identity proof and your recent photograph. Keep ready the photocopy of documents and your recent type photograph. Remember that you should submit photocopy of Identity card and Address proof of those person only for which the earlier micro/regular sim card was purchased.
Sim card seller or costumer care will make the new nano sim card activate within few minutes, but your old sim card will become inactive at the same time.
Before changing the sim card, it will be better to keep backup of all contact of your sim card. Also keep backup of SMS stored in your sim card and keep record of IMEI number of your previous sim card and new sim card.