Hello Anshul,
First of all, you should be passed in CBSE boards and you can only give an improvement of those subjects which you have passed..if you have failed in any subject you will have to give it's compartment..you can't give its improvement exam

In mid or last October there will be an update on Central Board of Secondary Education for improvement form click on that and fill the form..and complete they payment it's 200 per subject and 1000 asap..it will cost you around 2k for 5 subjects..then after filling from there will be a confirmation page..you need to send the confirmation page with candidate's sign on it to the address given on that page..then in February you will get the admit card by post on the address which you will fill in the form..and then.