How to write complex function

CourseService.php

<?php

class CourseService {
    
    ...
    
    public function signUp(SmartCode $code, User $user)
    {
        // 1. Check if the code is valid
    
        // 2. Check if the user already has a code for this course
    
        // 3. Register the code
    
        // 4. Enroll the user with his code
    
        // 5. Return enrollment
    }
    
    ...
}