First go to the Apennine mountains and ride the horse to where video is showing. you will find a bridge, move the horse as shown. It may take some time but keep trying, you will hit the right space soon. Now try to hop the horse down. After a while, it will hop straight down but if it does not, keep trying at the same spot. Finally you will fall down the bridge and under the map. If you step out the horse, it will let you float away. If you would keep the horse on bridge, it will let you riding the horse under the bridge as you can see in the video.