Google Cardboard DIY
Image from Tracey Tutt  Google Cardboard – VR out of a box!
Virtual reality from 78p
Here's how
If you are practical, patient and like making things then this is an interesting project

1. Buy parts
There are just a few parts you need which you can't really make.
The full specification is for lenses, magnets and "velcro" type sticky strips
The crucial bits are the lenses and you can get by with just these and make your viewer for 75p
The magnets and velcro make the experience better but are only viable to buy in multipacks
The magnets let you "click" things while in virtual reality ...  doesn't work with all phones & can be unreliable ... not essential but helps.
The velcro strips hold the flap where your phone goes ... you could use tape.

2 x  25mm diameter 45mm focal length Biconvex Lenses

1 x 20mm diameter round Ferrite magnet  e.g. FIRST4MAGNETS  19p (10 pack for £1.85 + £1.45 p&p)
1 x 20mm diameter round Neodymium Magnet e.g. StrongMagnetsDiscount 59p (40 pack for £23.58 inc p&p)

"Velcro pads"
 2 x 10mm x 25mm self adhesive hook and loop pads e.g. Hook and Loop Direct 18p (10 pack for 89p) 

2. Get some cardboard
A piece of cardboard 22cm x 56cm and 1.5mm thick
Amazon style boxes work well - many people use a large pizza box.

3. Print out the template
Download and print out the PDF template here
This is the simpler v1.2 version of Google cardboard

4. Cut out & assemble your cardboard viewer
Cut out the paper template, glue it to your cardboard, cut out your cardboard and assemble your viewer
This part can take anywhere between an hour and 3 hours!
Here are some great guides and videos how to do this
Revvrstudios  Building Google Cardboard from Scratch video goes over the whole process
Computerworld  DIY: Build your own Google Cardboard VR viewer slideshow guide
Android Authority DIY Google Cardboard (how to) video

Note - have the flat side of the lens so it faces you and the curved side facing the phone

Some go even further and show how to make a Google cardboard viewer for nothing - see the link below
Thaitrick Youtube video