I used to map read in the car, aged about 7, I think. I learned the techniques that I now know are called 'thumbing' and 'collecting features'. Thumbing being the same technique as used when you're learning to read; keep track of where you are on the page with your finger... 'Collecting features' being looking at the map and seeing what should be coming up next, and checking that it does...
I'm not sure how much tuition my DofE groups had had in map-reading (it seemed like almost no in-field practice), but, after the first morning, they had picked up enough of the basics that they didn't get too lost. The biggest problem in the first morning was generally excited chattering, rather than concentrating on the navigation task...
The list of topics Lorraine posted looks good, and you can follow it yourself; look at the map, look at the symbol page, and understand what they all mean. Then try to follow (on the map), a route you know well; it might simply be from you home to town. Starting with somewhere you know means that you should already have a good idea of what the real world looks like, and then you're learning how this is represented on the map.
The OS have some useful teaching resources, too, even if they are aimed at children.
There are some better articles that are archived.
Here's another map reding booklet.
There used to be an andvanced booklet:
http://www.ordnancesurvey.co.uk/oswebsite/mapshop/pdf/adv_map_reading.pdf
But this seems to have vanished. Google for the filename might bring up hits.