Since we charge only interest every month and principal in regular interval of 3/6 months, the principal payment is supposed to be done in lump sum amounts-which are bullet payments. It is calculated by dividing loan amount by number of 3/6 months in loan tenure.
For e.g. for a loan of Rs 1 Lakh and 60 months tenure, bullet amount applicable would be Rs 5,000 in every 3 months.