I would wait till the plant gets a couple nodes and a little bit of a beefy stem. But you can start really at any point in time. And LST is a rather easy concept and varies in degrees from a major full tie down to just 1 tie down. But to start you put a screw or hook in the side of the pot then 1 directly across from it. Tie a piece of string into a slip knot and put it over the plant and slide it all the way down to the stem bellow your first set of leaves. tie this to one of the screw without bending the plant at all but make sure the string dosnt have any give to it (this is going to be a counter point to stop the training for accidentally uprooting your plant). Then take a second string and tie a slip knot in it. slid this string over the plant to just below the top set of leaves (not including new growth). wrap the string around your screw and slowly pull the plant down till its almost horizontal in the pot.
hope this helps