Step::1 Visit the URL where you installed WHMCS.
Step::2 Add the admin folder name to the end of the URL. For example, if your admin folder name is ‘admin’ you would enter www.example.com/admin
Step::3 The Login page will appear, enter your admin username and password.
Step::4 Click Login to enter the admin area.
That's It !Check out the video tutorial to get detailed explanation on the topic. You may check out the blog as well.