So. 3 day walk in a few weeks time, with kids. Trying to teach them how to effectively plan. Given time to prepare, something like this can be invaluable from a military or even a purely civilian context.
Things you'll see marked;
OSGB36 in the bottom left corner - Datum for co-ordinates/GPS use.
SD/SE black line. British National Grid version of UTM 100k grid square letters. Important when giving a six figure grid reference (Again - Could be as simple as calling for Mountain Rescue...)
Vehicle routes in red - In this instance for danger - walking down small back country roads in the UK = Not a good plan. They often have walls/hedges either side and pin you ON the road. Our nation was not designed, nor does it have space, for cars
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Blue - Water features. Lakes, Rivers, Springs, Streams etc.
Black Crosshatching is built up areas - In this instance we want to avoid built up areas because the whole point is to go walking in the country. Your needs will vary
Green is wooded areas - I've marked them mostly because treelines make excellent navigational markers and are good to transit through of they're similar to the one shown here.
Brown Crosshatching is terrain which is going to be a pain in the arse - In this instance keeping the brown crosshatching on your right hand side means you're going in the right direction. For someone planning patrol routes it's a MASSIVE area obstacle.
Black triangle with a wavey line - Antennae - At night will have a light on it if it is marked on an OS map. EXCELLENT thing to get a GR for and use as a bearing to ascertain where on a line you are when you night nav.
4 corners are grid marked - So you can overlay it easily.
Contours (Index - Every 50m) I've started to mark these, particularly in areas of extreme change, to give an obvious idea of what the terrain will be like, and where the 'hard' terrain is.
Orange dashes - Intended route. If I were using this for patrols I would likely do my route planning on a seperate overlay - These can be combined to do 'honesty traces' - Check if you're using a bridge too often and it's likely to become an ambush area, or if you're not patrolling a whole portion of your AO. If there is a reason for it (Say, the sea....) then so be it. If there isn't it gives you the ability to reorg yourself to make sure you're doing it right.
Black dashes with 'escape' written - Because it's important to be able to get to a main road for ambulances to get medical personnel to you ASAP. They're marked so you don't have to think when someone gets hit. This is very much for health and safety purposes and hillwalking with teenagers. I would suggest these could also be a good idea if you have any form of vehicular backup or a medical team with you - Where can you casevac to that a vehicle can get to you ASAP (assuming you have one).
Tie Hawkeye's thoughts up with some pre planning (and remember - if you go out and find a river... MARK IT IN YOUR MAP AND ADD IT TO THE PLAN!) and this'll be easy as you like. You can practise the skills in breif if you go for a nice dawdle round the area with family, and that'll keep skills up for when you (if, hopefully never..) you decide something like this is actually strategically important....
Fresh socks. Goretex socks. Don't get goretex boots. Only one man needs to get wet to get everyone over. Yes. Yes I have 
