1. Create a new CloudFormation template file. We'll start by populating it with Parameters that correspond to all the requirements that we previously mentioned:

AWSTemplateFormatVersion: '2010-09-09' 
Parameters: 
  FullyQualifiedName: 
    Description: The fully qualified name for the directory
      (e.g. megacorp.com) 
    Type: String 
    AllowedPattern: '^([a-zA-Z0-9]+[\.-])+([a-zA-Z0-9])+$' 
  Password: 
    Description: The password for the directory Administrator 
    Type: String 
    NoEcho: true 

2. Then, we add parameters for the VPC, subnets, and directory size:

  VpcId: 
    Description: The ID of the VPC to deploy to 
    Type: AWS::EC2::VPC::Id 
  SubnetIds: 
    Description: Subnets where the directory will be deployed to
      (pick at least 2) 
    Type: List<AWS::EC2::Subnet::Id> 
  DirectorySize: 
    Description: The size of the directory to deploy 
    Type: String 
    AllowedValues: 
      - Small 
      - Large

3. Next, we define our Resources. Even though two Simple AD domain controllers are being deployed, we only need to create one resource here:

Resources: 
  ExampleDirectory: 
    Type: AWS::DirectoryService::SimpleAD 
    Properties: 
      Name: !Ref FullyQualifiedName 
      Password: !Ref Password 
      Size: !Ref DirectorySize 
      VpcSettings: 
        SubnetIds: 
          - !Select [ 0, Ref: SubnetIds ] 
          - !Select [ 1, Ref: SubnetIds ] 
        VpcId: !Ref VpcId

4. You can now go ahead and run this template in the CloudFormation web console, or via the CLI, like this:

aws cloudformation create-stack  
  --stack-name example-directory  
  --template-body file://08-active-directory-as-a-service.yaml  
  --parameters  
  ParameterKey=FullyQualifiedName,ParameterValue=<fqdn>  
  ParameterKey=Password,ParameterValue=<password>  
  ParameterKey=VpcId,ParameterValue=<vpd-id>  
  "ParameterKey=SubnetIds,ParameterValue='<subnet-1>,<subnet-2>'"  
  ParameterKey=DirectorySize,ParameterValue=<Small/Large>