Agreed, SAS handbook is a god send when making lesson plans. Doesn't tell anything about the mod/artic tents though...
Basic set up for lean to:
1. Select trees to be used.
2. Tie the crossbeam at waist height for the tallest person in the group and test it. Test is best done by having largest person sit/jump on it.
3. Place the ribs every 1-1.5 feet or so. Closer if you can. At this point cover the lean to with a tarp/groundsheet if you are using one.
4. Using easily bent sticks weave in and out between the struts to provide a base for the pine boughs.
5. Place the pine boughs in alternating order, cruve up, then curve down. This will create a pocket of air to help with insulation. Make sure you stat at the bottom and work your way up so that the water won't flow down into your shelter. Continue placing the boughs until they are thick and full. Good guide is if you stick your arm in and the pine come up to your elbow.
That's a basic idea of how to do a lean to, next an A-frame.
1. Find your main spar and the first two ribs that will help support it.
2. Place them is an "A" formation and tie them solid. Once again, the top should be at about the waist of the tallest person. Test as for lean to.
3. Place ribs along both sides about 1-1.5 feet apart, closer if possible.
4. Follow steps as for lean to above.
These should be faily accurate. I'm sure one of the SI's here can provide a better descripton...