It’s very simple. Just redirect to https://developer.salesforce.com/signup and fill all details and click on ‘Sign me up’.
You will receive an email to setup the password. Just follow the instruction and you are good to create a developer edition org.
Salesforce has implemented a restriction on creating accounts using a unique email address. If you try to enroll with an email address that is already in use, you will receive an email notifying you that you cannot proceed. If you wish to sign up with a different email address, you can visit the following link: https://developer.salesforce.com/form/signup/freetrial.jsp.
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