Example CloudFormation stack building two public subnets.

PublicSubnets.yaml

---
AWSTemplateFormatVersion: '2010-09-09'
Description:
  The Public Subnet, and associated routing information
  
# Metadata: # no metadata

Parameters:
  Environment:
    Type: String
    Description:
      Stack Environment Prefix.

  PrimaryAvailabilityZone:
    Type: AWS::EC2::AvailabilityZone::Name
    Default: us-east-1a # Probably shouldn't set a default, as it makes this region dependent

  SecondaryAvailabilityZone:
    Type: AWS::EC2::AvailabilityZone::Name
    Default: us-east-1b # Probably shouldn't set a default, as it makes this region dependent

#Mappings:
  
# Conditions: # No Conditions at this time.

# Transform: # No Transforms at this time

Resources:

  # We need to create a VPC Gateway, and then attach it to the VPC.  
  VPCGateway:
    # Using an Internet Gateway for now; may change to a VPN gateway if needed, but one step at a time.
    Type: AWS::EC2::InternetGateway
    Properties:
      Tags:
        - Key: Name
          Value: !Sub "${Environment} VPC Internet Gateway"

  VPCGatewayAttachment:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      InternetGatewayId: !Ref VPCGateway
      VpcId: 
        Fn::ImportValue: !Sub "${Environment}::VPC"

  # We need a subnet for publicly available servers. We need two, so that we can register
  # a load balancer.
  PublicSubnetAZ1:
    Type: AWS::EC2::Subnet
    Properties:
      CidrBlock: 10.0.1.0/24        # 10.0.10.0 -> 10.0.1.255
      MapPublicIpOnLaunch: false    # We will use elastic IPs for public-facing servers.
      AvailabilityZone: !Ref PrimaryAvailabilityZone
      VpcId: 
        Fn::ImportValue: !Sub "${Environment}::VPC"
      Tags:
        - Key: Name
          Value: !Sub "${Environment} Public Subnet AZ1"

  PublicSubnetAZ2:
    Type: AWS::EC2::Subnet
    Properties:
      CidrBlock: 10.0.2.0/24        # 10.0.20.0 -> 10.0.2.255
      MapPublicIpOnLaunch: false    # We will use elastic IPs for public-facing servers.
      AvailabilityZone: !Ref SecondaryAvailabilityZone
      VpcId: 
        Fn::ImportValue: !Sub "${Environment}::VPC"
      Tags:
        - Key: Name
          Value: !Sub "${Environment} Public Subnet AZ2"

  # In order for subnets to receive traffic from the public, we need to create
  # routing tables and rules.
  PublicRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: 
        Fn::ImportValue: !Sub "${Environment}::VPC"
      Tags:
        - Key: Name
          Value: !Sub "${Environment} Public Route Table"

  PublicSubnetRoute:
    Type: AWS::EC2::Route
    Properties:
      RouteTableId: !Ref PublicRouteTable
      DestinationCidrBlock: 0.0.0.0/0 # We have no idea what IPs may be assigned; got to go global
      GatewayId: !Ref VPCGateway
    # The route can not be configured until the gateway is attached to the subnet.
    DependsOn: VPCGatewayAttachment

  PublicRouteTableAssociationAZ1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnetAZ1
      RouteTableId: !Ref PublicRouteTable

  PublicRouteTableAssociationAZ2:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnetAZ2
      RouteTableId: !Ref PublicRouteTable
      
      
Outputs:
  PublicSubnetAZ1:
    Description: The publicly accessible subnet
    Value: !Ref PublicSubnetAZ1
    Export:
      Name: !Sub "${Environment}::PublicSubnetAZ1"

  PublicSubnetAZ2:
    Description: The publicly accessible subnet
    Value: !Ref PublicSubnetAZ2
    Export:
      Name: !Sub "${Environment}::PublicSubnetAZ2"

A example of creating public subnets using CloudFormation. See https://twasink.net/2019/01/15/ail-subnets-public-subnets-wow-they-are-free-and-we-can-au-to-mate/