Google released it's DIY VR headset with a clear cut instruction as to how to make at home with a pizza box. We made one, in fact many and thought will share the experience.
The above link gives clear instructions on how to make one and even manufacture if you intend to commercially sell it.
This instructable however, focuses on the limitations while making one at home.
Step 1: Print the Template
This shouldn't really be an issue unless you don't have a cutter, scale and some cardboard boxes.
The only painful thing at this stage is cutting those slits and the circles for the lenses.
Using circle cutter will make the job easier. If you are not equipped with one you can place a coin and use it as a guide to the cutter.
Step 2: The Lenses
Google recommends a 25mm dia and 45mm focal length biconvex lens. You can order a pair of them from amazon or other online stores.
In case you are in a country where amazon doesn't ship or you just cannot wait for the shipping to make one, then you can walk to a stationery store and ask for a pair of lousy binoculars.
Remove the object lenses (not the eye piece) and use them.
Finding the focal length.
The tricky part is the focal length. It might be more than 45mm. In that case find the focal length by converging the sun's image to dot on a paper. The distance between the lens the and paper is the focal length. If the sun had set by the time you reached this stage then use a distant light source about 3 meters away.
Now place the phone at a distance equal to the focal length from the lens. Modify the cardboard to accommodate the distance. Look through the lens and place it at a distance till you get a blur-free image of the screen.
If you look through your cardboard you might see two images of the same object slightly offset(refer avatar photo). Then the principle axes of your lenses are not parallel. Slightly adjusting them will result in a clear single image with the depth popping out.
Step 3: The Magnets, NFC Tag and Velcro
NFC tag is used to launch the cardboard app automatically. If you don't mind starting the app manually and then inserting in the cardboard then you can do away with the NFC. Similarly the Velcro is used to hold the phone in place. The same thing can be achieved with a scotch tape or by just holding the contraption in hand firmly.
Google recommends neodymium magnets. however, the one that sticks on your refrigerator will work fine. Again, the magnets are also optional. They are used to make selections while you are viewing through the cardboard. These selections can be made by just tapping the screen.
Step 4: Cardboard Variants...
Once you understand how the cardboard works you can make variants with minimum resources.
Step 5: Durovis Dive
When you have sufficient resources at your disposal you can take it a step ahead.
We 3d printed this open source VR headset from the site http://www.durovis.com/opendive.html.
Added some cushion and elastic strap for hands-free and comfortable viewing.
Step 6: Download Vr Apps
There are a truck load of VR and cardboard apps. Also some SBS(side by side) players to watch 3d movies.
You can check the following link.
Vanguard V is one of my favorites.