A CloudFormation test template to create a VPC with a public subnet and spin up a internet connected instance inside of it.

cloudformation-vpc-from-scratch.json

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Description": "Building a VPC from scratch with CloudFormation",

    "Mappings": {
	"SubnetConfig": {
	    "VPC":    { "CIDR": "10.0.0.0/16" },
	    "Public": { "CIDR": "10.0.0.0/24" }
	},
	"RegionMap" : {
	    "eu-west-1": { "AMI" : "ami-8e987ef9" }
	}
    },

    "Resources": {
	"VPC": {
	    "Type" : "AWS::EC2::VPC",
	    "Properties" : {
		"CidrBlock" : { "Fn::FindInMap": ["SubnetConfig", "VPC", "CIDR"] },
		"EnableDnsSupport" : "true",
		"EnableDnsHostnames": "true",
		"Tags" : [ { "Key": "Application", "Value": { "Ref": "AWS::StackName"}},
			   { "Key": "Network", "Value": "Public"}
			 ]
	    }
	},
	"PublicSubnet": {
	    "Type" : "AWS::EC2::Subnet",
	    "Properties" : {
		"CidrBlock" : { "Fn::FindInMap": ["SubnetConfig", "Public", "CIDR"] },
		"Tags" : [ { "Key": "Application", "Value": { "Ref": "AWS::StackName"}},
			   { "Key": "Network", "Value": "Public"}
			 ],
		"VpcId" : { "Ref" : "VPC" }
	    }
	},
	"InternetGateway": {
	    "Type" : "AWS::EC2::InternetGateway"
	},
	"GatewayToInternet": {
	    "Type" : "AWS::EC2::VPCGatewayAttachment",
	    "Properties" : {
		"InternetGatewayId" : { "Ref": "InternetGateway" },
		"VpcId" : { "Ref": "VPC" }
	    }
	},
	"PublicRouteTable": {
	    "Type" : "AWS::EC2::RouteTable",
	    "Properties" : {
		"VpcId" : { "Ref": "VPC" }
	    }
	},
	"PublicRoute": {
	    "Type" : "AWS::EC2::Route",
	    "DependsOn": "GatewayToInternet",
	    "Properties" : {
		"DestinationCidrBlock" : "0.0.0.0/0",
		"GatewayId" : { "Ref": "InternetGateway"},
		"RouteTableId" : { "Ref": "PublicRouteTable" }
	    }
	},
	"PublicSubnetRouteTableAssociation": {
	    "Type" : "AWS::EC2::SubnetRouteTableAssociation",
	    "Properties" : {
		"RouteTableId" : { "Ref": "PublicRouteTable" },
		"SubnetId" : { "Ref": "PublicSubnet" }
	    }
	},
	"PublicInstance": {
	    "Type" : "AWS::EC2::Instance",
	    "DependsOn": "GatewayToInternet",
	    "Properties" : {
		"ImageId" : { "Fn::FindInMap" : [ "RegionMap", { "Ref" : "AWS::Region" }, "AMI" ]},
		"InstanceType" : "t1.micro",
		"NetworkInterfaces" : [ { "AssociatePublicIpAddress": "true",
					  "DeviceIndex": "0",
					  "DeleteOnTermination": "true",
					  "SubnetId" : { "Ref": "PublicSubnet" } } ]
	    }
	}
    }
}