Since “Landscape mode” is the horizontal orientation of your page, meaning that the screen has more width to accommodate your content.
You can simply drag the handles to increase the width of the page and see how it looks like in landscape orientation, but you need to be aware of the screen size of the device you are using.
Also, please remember that on smaller devices, it would be better to avoid using VH unit and use PX and % instead.
Do not forget of dockings also, it could help you a lot in the creation process.