How to write a complex function

CourseService.php

<?php

class CourseService {
    
    ...
    
    public function signUp(SmartCode $code, User $user)
    {
        $this->guardThatSmartCodeIsValid($code);
        
        $course = $code->getPayload();

        $this->guardThatUserIsNotSignedUp($user, $course);
        
        $this->smartCodeService->register($user, $code);

        $enrollment = $this->smartCodeEnrollment->enroll($user, $course);
        
        return $enrollment;
    }
    
    ...
}