You need to be registered in the ECFMG first. Once you have your ECFMG number and password you can proceed. Start a Step 1 application, fill the info step by step. The info you need to fill is mostly about your personal info, school, graduation date...etc. Make sure you spell your name correctly and the choose the correct title for you medical degree. ie MBBS vs MD. Then you have to do the payment thru a credit card or check. You'll have to print a certain form at the end, fill it up, enclose a personal picture, stamp it from you deanship, and send it to the ECFMG by mail.
You will be notified by e-mail after each step, and ECFMG will tell you always what to expect and what to do.
Best of luck 🙂