Until recently I used Eclipse, and in Eclipse I simply created a folder /res/layout-landscape and put alternate landscape layouts inside.
However, it seem in Android
You can actually use layout-land in Android Studio as well it will show up in your res directory like so:

If it doesn't show up make sure to have your project scope set to Project and not Android.

If you still don't see one you can create one by right clicking on the res directory and selecting New -> Android resource directory and filling out the prompt below.

Also, once it's been created, you can easily switch from portrait to landscape in the layout preview via the orientation button found on the toolbar above the layout preview. It is pictured below:
