1. Sign Up
Use the following link to register: Developer Edition Sign Up
If you already have a Developer Edition account associated to your email address, you can create a second account by choosing a username that is different from your email address (username does need to be in the format of an eamil address, however).
2. Verify Your Account
Click the verify link sent your email inbox from Salesforce
3. Verify Your Password
4. Navigate to Setup
(If you don’t start on this page)
5. Enable "Administrators Can Log in as Any User"
This will allow you to test multiple user accounts without using separate usernames/passwords.
6. Disable "Force relogin after Login-As-User"
This will allow you to test multiple user accounts without logging in every time.
7. Optional: Update Your Password Policy
If you don’t want your password to expire in your Developer Edition account, you can set passwords to never expire. This is certainly not recommended for production environments.