When and how should i start LST-Training

I would say, don't bend the top for now. Looking at the side of the plant, your nodes are pretty tight. I'd start with pulling the branches out and go from there. The top may end up growing much faster and then you can tie it down as long as the stem doesn't get super hard. I had to do it that way with a plant I'm currently growing. Worked a charm.

I think now is exactly the right time for that. Make sure not to break the main stem. Attach the stem with a wire (loop) to a pot. It doesn't have to be tight. Then carefully bend the main stem vertically parallel to the ground so that the lower parts of the plant get more light. You can secure the upper part well with U-shaped stakes in the soil. With your size, you can do this with 1-2 other branches as well. Check out my diary.

A stem eventualyl gets rigid and not easily bent... anytime before that point is okay to train. Your plant is stocky, so i'd just work on shaping the branches vs trying to bend it over. if the nodes are too tight, reduce light slightly. You need some internode to be able to train them properly or it's all bunched up and a disease vector later on when it fills in.