First, click the 'Invite' link under the hiring step the applicant is in. For example, the applicant below has completed his assessment, so he is in the Phone Screen hiring step and should be invited for a phone screen. However, if he is an exceptional applicant, you do have the option to skip the phone screen step and invite him for an in-person interview.
When you click either of these options, an email template will open. Below is the template for the phone screen invitation.
Notice that the applicant's name and the job name immediately populate, so you don't need to edit anything. Make sure to click Send when you're ready to send the email.
Learn how to edit to these default email templates: How do I customize the email templates that are sent to applicants?