Dear Pooja, for IELTS, you need to visit their official website: www.ieltsidpindia.com and click on Register for IELTS. Based on the availability status, you can book the date and the city you want to take the exam on. Fill in the application form and make the payment. Once you complete the payment and the date and slot are confirmed, you need to courrier the same to IDP Head Office. You can check more information at their official page at: https://www.ieltsidpindia.com/information/how-to-register?gclid=CjwKCAjwxaXtBRBbEiwAPqPxcMiLieYwNqhi8paScIcIW-P5_X0T-GI5cMvUwaQvX-f-ZsWL-PRTzRoClNEQAvD_BwE
For GRE, the procedure is almost the same. You need to have a ETS Account. Once you visit the official website, you need to select if you want to choose GRE General or Subject. Select the date and time slots and fill in the application form and make the payment.
The both exams you mentioned are for the foreign countries studies Candidates need to create an ETS account. Selec the type of GRE exam you want to take GRE General or GRE Subject Test. Select the date when you want to give the GRE Exam and find the nearest test center. Give your academic details. Proceed ahead and pay registration fee $205.
all the best