Google Cardboard Kit
Virtual reality from £2.45
Here's how

1. Buy Your kit
Most of the kits are very similar if not the same so you may as well try the cheapest
They usually come from China so allow up to 3 weeks for delivery.
Amazon: Koly DIY Google Cardboard = £2.45  (£1.25 + £1.20 P&P)
Amazon: Full Time 6 inch DIY For Google Cardboard = £2.39  (£1.59 + £0.80 P&P) 
Amazon: TaylorRoco Virtual Visual = £2.41 (P&P included)

2. Assemble Your kit 
This is easier than it seems and can be done in just a few minutes
There are plenty of videos on Youtube of "how to assemble google cardboard
Here are just a few

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