S.13  MOVING SUPPLY TRAINS
There are three different actions for moving supply trains around the map. The number of that can be moved in a single action is limited by your supply capacity. There is no cost in BPs for moving supply trains. The "ALL" and "ALL BUT" formats work in all these cases.
Note: You don't have to own the area where you move supplies with the WAGONS or UNLOAD actions, so you can use these  to send supplies to another empire (deliberately, or otherwise).

S.13.1  SIMPLE SUPPLY MOVE
On land a WAGONS action can move supply trains from one land area you own to an adjacent land area (whether you own it or not). This also resets the retreat location of the destination area if you own it (but doesn't set the retreat mode - so you can use this action to set a retreat location without setting the defence mode to retreat at the same time).
[ WAGONS ]  [ WHERE FROM ]  [ WHERE TO]  [ NUMBER ]   

S.13.2  USING SUPPLY LINES
You can use the SEND action to move supply trains "up" a supply line (a chain of retreat locations - see S.4) from a supply centre to any land area that's on a supply line starting from that supply centre (ie. any area that shows this supply centre as where it draws supplies).
[ SEND ]  [ WHERE FROM ]  [ WHERE TO]  [ NUMBER ]   
Note: You can also send supplies from another supply centre, if there's a supply route from that supply centre to the normal supply source for the area (ie. supply lines chain together).

S.13.3  SUPPLY BY SEA
Supply trains can be moved by sea from a naval base to a land area adjacent to any ships that come from that base (provided that base isn't blockaded) by using the UNLOAD action. This is ordered by entering the code for the sea space in the "where from" box and the adjacent land area in the "where to" box (your orders don't say what naval base is involved because we know what naval base the supplies should be in - because we keep track of which base the ships came from). The number that can be moved this way is limited by the number of ships.
[ UNLOAD ]  [ WHERE FROM ]  [ WHERE TO]  [ NUMBER ]   
Note: This is the only way that supply trains can be moved by sea. Your fleet can "instantly" fetch supplies from their naval base and throw them ashore.

ABOUT SUPPLY LINES
For supply trains to be any use you've got to move them to the area you want them, which will be wherever your armies are located. It's very quick and easy (and cheap) if you've got a supply line running back to a supply centre through areas you control (or if your navy is on the coast).
If someone breaks your supply lines then your armies will soon be unable to move against the enemy and fight, and you have the choice of bringing up another army to remove the obstruction or shifting your lines (by changing retreat locations and/or by using WAGON actions).
Ships don't need supply lines, because ships can carry their own supplies. Armies being moved at sea are also supplied by their own ships (they're carried on transports, not by the escorting warships) and you don't have to worry about providing supplies until after they go ashore.