first of all this time improvement form will be available in 3rd week of October.when the form is available then you have to open that link and fill your board roll no. and year of passing. after that you have to verify your details and then it will ask which subjects you want to apply for improvement. in this you can choose all subjects or a particular subject of your choice then you have to pay some amount for the improvement form. i think you have to generate a challan and pay it in a bank after that you have to send some documents to cbse regional office. the documents are your acknowledgement page, 12th marksheet xerox ,challan xerox then you have to download your admit card which will be available in month of February. you will get your marksheet at your address by post so please fill the correct address in form because according to that you will get the exam centre and regional office .