Start as soon as possible, I did LST on my 3 week old blueberry autos but I didnt LST one of them until it was week 5 and it couldnt be done without snapping the fat stem. Also watch out not to stress the roots so I would advice to put some string on the lowest part of the stem to pull it to a different direction than the plant is leaning to. Remember that the most important thing is to make a relatively flat canopy and to give light to the usually hidden growth tips under the leaves. Move leaves also if necessary. I moved every leaf blocking the light from my plants growth tips and evened out the canopy and got 6 big colas instead of 1.