CloudFormation: Template Anatomy and a VPC Example

A template has a few sections: Parameters, Mappings, Conditions, Resources (required), and Outputs.

AWSTemplateFormatVersion: "2010-09-09"
Parameters:
  Env: { Type: String, AllowedValues: [dev, prod], Default: dev }
  VpcCidr: { Type: String, Default: 10.20.0.0/16 }
Conditions:
  IsProd: !Equals [!Ref Env, prod]
Resources:
  Vpc:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: !Ref VpcCidr
      EnableDnsHostnames: true
      Tags: [{ Key: Name, Value: !Sub "${Env}-vpc" }]
  PrivateSubnet:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref Vpc
      CidrBlock: !Select [0, !Cidr [!Ref VpcCidr, 4, 8]]
      AvailabilityZone: !Select [0, !GetAZs ""]
Outputs:
  VpcId:
    Value: !Ref Vpc
    Export: { Name: !Sub "${Env}-vpc-id" }

pip install cfn-lint && cfn-lint template.yaml
aws cloudformation deploy --template-file template.yaml --stack-name dev-network \
  --parameter-overrides Env=dev --region ap-south-1

Ref, Sub, GetAtt, Select and If cover most needs. Exported outputs let other stacks import values with Fn::ImportValue.