How can i use a renderer

I did this xaml in my toolbar

    <?xml version="1.0" encoding="utf-8"?>
    <android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar"
        android:popupTheme="@style/ThemeOverlay.AppCompat.Light">
        <LinearLayout
            android:id="@+id/alternateTitle"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal">
            <ImageView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="start"
                android:id="@+id/toolbarIcon" />
            <TextView
                android:id="@+id/alternateTitle1"
                android:text=""
                android:textSize="@dimen/notification_subtext_size"
                android:textColor="@color/material_deep_teal_500"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textAppearance="?android:attr/textAppearanceLarge" />
            <TextView
                android:id="@+id/alternateTitle2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
        </LinearLayout>
    </android.support.v7.widget.Toolbar>

and then i did this renderer

[assembly: ExportRenderer(typeof(NavigationPage), typeof(NavigationRenderer))]
namespace Operacional.Droid
{
    public class NavigationRenderer : NavigationPageRenderer
    {
        private Support.Toolbar _toolbar;

        public override void OnViewAdded(Android.Views.View child)
        {
            base.OnViewAdded(child);

            if (child.GetType() == typeof(Support.Toolbar))
                _toolbar = (Support.Toolbar)child;
        }

        protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);

            var page = this.Element as NavigationPage;

            if (page != null && _toolbar != null)
            {
                Typeface tf = Typeface.CreateFromAsset(Android.App.Application.Context.Assets, "Arial.ttf");

                TextView title = (TextView)_toolbar.FindViewById(Resource.Id.alternateTitle1);
                title.SetText(page.CurrentPage.Title, TextView.BufferType.Normal);
                title.SetTypeface(tf, TypefaceStyle.Normal);
                title.Text = "Paulo Correa";
            }

        }

    }
}

and how can i make to work this renderer? How can i changes text, font, size and so on? How do i call the renderer in my project? Do i call in the App.cs or where?

i'm reading this

https://developer.xamarin.com/guides/xamarin-forms/application-fundamentals/custom-renderer/