Step 1: Folding Step 1
Make a fold across two of the opposite corners. It should now look like the picture.
Next fold up the long straight about 1cm at a time up to about where it is in the picture. Make sure it is flatttened.
Step 2: Folding Step 2
Next you will need a hard edge, the edge of a table or chair is perfect for this. Hold the paper by the two ends and scrap it up and down against the hard edge as shown in the picture. This makes the paper able to then form a curve which will become the leading edge of the ring wing.
Lastly take the two ends and tuck one (pick the thinnest one) into the other one. If you have an issue with the two ends staying together it is ok to use a small piece of tape to hold them together if it is necessary. This should now form a ring as shown in the main picture.
Step 3: Testing and Flying
now for the fun bit.
Hold the plane vertically between your forefinger and ring finger as shown in the picture. To launch it you need to rotate your wrist while pushing the plane forwards. When you release the plane it should be horizontal.
There is a link here to a video to give you the idea.