This is a quick and dirty example i made that uses my own validation on a property field.

View

<div ng-controller="My.MarkdownEditorController" class="umb-editor">
    <input type="number" ng-model="model.value" id="{{model.alias}}" val-property-validator="myValidateFunction" class="umb-editor" name="myField" />
    <br />

    <span class="help-inline" val-msg-for="myField" val-toggle-msg="myCustomValidationKey" 
          style="display: none;">Invalid</span>
</div>

Controller

angular.module("umbraco")
    .controller("My.MarkdownEditorController",
    function ($scope) {


    $scope.myValidateFunction = function () {

        //set isValid too true so that if this property isnt marked as mandatory on the document type the property is allways valid
        var isValid = true;

        if ($scope.model.validation.mandatory) {
            var inputValue = $scope.model.value;
            var intValue = parseInt(inputValue);
            isValid = intValue % 2 === 0;
        }

        return {
            isValid: isValid,
            errorMsg: "Value cannot be empty",
            errorKey: "myCustomValidationKey"
        };
    }

});

Manifest

{   
    //you can define multiple editors   
    propertyEditors: [      
        {
            /*this must be a unique alias*/ 
            alias: "My.MarkdownEditor",
            /*the name*/
            name: "Markdown editor",
            /*the html file we will load for the editor*/
            editor: {
                view: "~/App_Plugins/MarkDownEditor/markdowneditor.html"
            }
        }
    ]
    ,
    //array of files we want to inject into the application on app_start
    javascript: [
        '~/App_Plugins/MarkDownEditor/markdowneditor.controller.js'
    ]
}

You can also check the Umbraco Source Code for an example https://github.com/umbraco/Umbraco-CMS/tree/9290f61f9485c5bef0aa9a95e05e8acefa9ce221/src/Umbraco.Web.UI.Client/src/views/propertyeditors/tags

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