Here is an example which we used for the Blog

public class BlogSave : ApplicationEventHandler
{
    protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
    {
        //Add event to allow searching by site section
        LogHelper.Info<BlogSave>("Attatching BlogSaveHandler to ExamineManager.Instance.IndexProviderCollection[\"ExternalIndexer\"].DocumentWriting");
        var indexerSite = (UmbracoContentIndexer)ExamineManager.Instance.IndexProviderCollection["ExternalIndexer"];
        var helper = new UmbracoHelper(UmbracoContext.Current);
        indexerSite.DocumentWriting += (sender,e) => IndexerSite_DocumentWriting(sender, e, helper);
        LogHelper.Info<BlogSave>("BlogSaveHandler attached to ExamineManager.Instance.IndexProviderCollection[\"ExternalIndexer\"].DocumentWriting");
    }

    private void IndexerSite_DocumentWriting(object sender, Examine.LuceneEngine.DocumentWritingEventArgs e, UmbracoHelper helper)
    {
        var tagsOnNode = helper.TagQuery.GetTagsForEntity(e.NodeId).Where(t => t.Group == "Blog");
        LogHelper.Debug<BlogSave>($"Addings tags for node:{e.NodeId} to field:tagList");
        foreach (var tag in tagsOnNode)
        {
            e.Document.Add(new Field("tagList", tag.Text, Field.Store.YES, Field.Index.ANALYZED));
            LogHelper.Debug<BlogSave>($"Tag: {tag.Text} for node:{e.NodeId} added to field:tagList");
        }
    }
}