Here's a simple example of using cloud-init to automate the configuration of an instance on first boot:

```yaml
#cloud-config

# Update packages on first boot
package_update: true
package_upgrade: true

# Install additional packages
packages:
  - nginx
  - php-fpm

# Write files to the system
write_files:
  - path: /var/www/html/index.php
    content: |
      <?php
      phpinfo();
      ?>

# Run commands on first boot
runcmd:
  - systemctl start nginx
  - systemctl enable nginx

# Create a user
users:
  - name: webadmin
    groups: sudo
    shell: /bin/bash
    sudo: ['ALL=(ALL) NOPASSWD:ALL']
    ssh_authorized_keys:
      - ssh-rsa AAAAB3NzaC1yc2EAA...your_public_ssh_key_here

# Configure SSH access
ssh_pwauth: false
disable_root: true
```

In this example:

1. The `package_update` and `package_upgrade` directives ensure that the system packages are updated on first boot.

2. The `packages` section specifies additional packages to be installed, in this case, `nginx` and `php-fpm`.

3. The `write_files` section is used to create a file on the system. Here, it creates a simple PHP script at `/var/www/html/index.php`.

4. The `runcmd` section specifies commands to be executed on first boot. In this case, it starts and enables the Nginx service.

5. The `users` section is used to create a user named `webadmin` with sudo privileges and an authorized SSH key.

6. The `ssh_pwauth` and `disable_root` directives are used to configure SSH access, disabling password authentication and root login.

To use this cloud-init configuration, you would save it as a YAML file (e.g., `cloud-config.yaml`) and provide it to your cloud provider or provisioning tool when launching a new instance.

Cloud-init will execute the specified configuration on the instance's first boot, automating the process of updating packages, installing software, creating files and users, and configuring SSH access.

This is just a simple example, but cloud-init supports a wide range of directives and modules for configuring various aspects of an instance, such as networking, storage, and more.