Navigate to https://trailhead.salesforce.com/

Click on the "Email" icon to create a login using your email address.  Similarly, you can login using Google, LinkedIn, Apple ID, or Facebook, if you already have an account associated with any of those services. 

Enter your email address in the Email Address field and click on "Log In". I highly advise  that you to use a personal email account, instead of an email account associated with your place of employment.  This will allow you to maintain access to your Trailhead account even if you lose access to the email associated with your organization. 

Trailhead will send you an email with the single-use Trailblazer.me code.   

Go to your email and obtain the single-use code from Trailblazer, allowing you to login. 

Enter the code you received from Trailblazer.  The code is unique and your code will be different. 

Trailblazer  could not find an account associate with my email address.  Click on "Sign Up".

Enter your First Name, Last Name, and create a personalized  URL                                  for your Trailblazer profile.  I will set Public Profile to "Off" for this account.  However, you may wish to keep it on. 

Click on "Done".  This will complete your profile. 

Start your tour of Trailhead by clicking on the "App Launcher". 

How to Set Up a Salesforce Trailblazer Account