Old windows are fairly easy to find. Salvage yards and second hand stores often carry antique looking windows. Find windows that have a thick enough frame for eye hooks to screw in without breaking the glass.
The windows hang from a simple support made from 4x4 posts: two that sit vertically in the ground and one piece that connects the two tall posts. Choose posts that are naturally rot resistant, such as redwood, pressure-treated lumber or cedar. The frame should be at least 6' tall and will need to be in the ground about 2' deep, therefore, 8' posts will work. Cut the top piece of the frame depending on the width of the window and the space you’d like between the window and the posts.
Stain posts with an outdoor wood stain. Let dry for several hours.
Using a posthole digger or auger, dig two holes about 2' deep. Space the holes according to how wide the window is and the desired space between the window and the support posts of the frame.
Set the stained 8' length (or desired height) posts in holes. Make sure posts sit level and then add water to concrete in hole. Let dry overnight.
Once the concrete is dry and posts have set up overnight, cut the posts at the same height. Use a string level to insure that they are cut level to one another. Use a circular saw to cut each post.
Add the 4x4 to the top of the cut support posts. This creates a frame for the window to hang. Toenail the 4x4 into the support posts, then using an additional piece of small lumber, secure screws into the 1x4 material into the frame joints to create a solid frame.
You can hang two windows from a frame or just one. For a single window, the hardware assembly is as follows: screw eye bolts to the top and bottom of each frame. Make sure the eye bolt is not too long that it breaks the glass. The eye bolts on the bottom are optional, they are there if you’d like to add a window weight to keep the window from swinging in the wind. Using two long lengths cable, loop the first piece around the top left eye bolt and use a cable clamp to secure the loop. Repeat on top right eye bolt. Make a loop at the loose end of each piece of the cable to attach to the eye bolt in the window support post. Loosely clamp the loop in place. Each length of cable should be about even, but you will have to adjust once it’s hanging. Connect a carabiner to the top loops on both sides.
If you want to hang two-stacked windows, repeat the steps above. Connect the windows through eye hooks, two lengths of cable, and create loops to string through each eye hook.
Screw eye bolts into the 4x4 top piece of frame spaced at the same width as they are set on the window.
Connect the carabiners attached to the top loops on the windows to the eye hooks in the support post frame. Make sure the windows set level by adjusting the size of the loop and adjusting the cable clamp.