The below link shows you exactly step by step process to fill the jeemain application form: https://engineering.careers360.com/articles/how-to-fill-jee-main-application-form
hope this helps.
Follow the below steps to fill the IIT JEE Main application form.
fill JEE Main registration form.
fill the detailed JEE Mains application form
upload the scanned image of their passport size colored photograph and signature in JEE Main application form 2021.
Application fee payment
Please refer to the below given link where you will find detailed explanation on the steps for filling up the application form: