CustomLayoutResolver.cs

namespace Helix.Skeleton.Feature.CustomLayoutResolver.Processors
{
    using Helix.Skeleton.Feature.CustomLayoutResolver.Constants;
    using Sitecore.Data.Items;
    using Sitecore.Pipelines.HttpRequest;

    public class CustomLayoutResolver : Sitecore.Pipelines.HttpRequest.LayoutResolver
    {
        public override void Process(HttpRequestArgs args)
        {
            // CustomLayoutSettings.LayoutItemId is the ID of the new layout item in Sitecore
            LayoutItem layoutItem = Sitecore.Context.Database.GetItem(CustomLayoutSettings.LayoutItemId);
            Sitecore.Context.Page.FilePath = layoutItem.FilePath;
        }
    }
}