"Resources" : {
...
"DBInstance" : {
  "Type": "AWS::RDS::DBInstance",
      "Properties": {
        "DBName" : { "Ref" : "DBName" },
        "Engine" : "MySQL",
        "MasterUsername" : { "Ref" : "DBUsername" },
        "DBInstanceClass" : { "Ref" : "DBClass" },
        "DBSecurityGroups" : [{ "Ref" : "DBSecurityGroup" }],
        "AllocatedStorage" : { "Ref" : "DBAllocatedStorage" },
        "MasterUserPassword": { "Ref" : "DBPassword" }
      }
},
"DBSecurityGroup": {
  "Type": "AWS::RDS::DBSecurityGroup",
  "Properties": {
     "DBSecurityGroupIngress": { "EC2SecurityGroupName": { "Ref": "WebServerSecurityGroup"} },
     "GroupDescription" : "Frontend Access"
  }
},
  ...
},