You must enter the following link: https://wallet.testnet.near.org/ and Click on the button "Create Account".
You must enter an account name and verify that it is available and click in "Reserve My Account Id".
Now you should choose the type of security for the account, in this case we will choose "Secure Passphrase".
All 12 secret words will now be displayed, write down the following words in order and keep them somewhere safe. Anyone with access to it will also have access to your account! You’ll be asked to verify your passphrase next.
Now you must randomly enter one of those words.
At the end of these steps you will be able to access your new Testnet Account.