LST is simply about manipulating apical dominance. Highest growth generally gets more resources and grows faster -- primary vs secondary branching is part of that calculation too.
Growth should spread out along the horizontal stretchin of stem... bend things down temporarily to allow other branches to catch up as needed.
With an autoflower, i wouldn't top. You have an unknown legnth of vegetative growth, so i'd shoot for ~7 primary branches and let the top start to grow vertically -- it will catch up if behind a bit, too.. so let the other growth get a few inches ahead of the top cola before letting it grow vertically. Secure that branch to remain horiztontal up until that point. If you don't tie down the end, it will curve upward shortly after your plant bender. Those lowest 2 axilla tend to be trash, i'd pinch those off before they waste resources.
Since you'll have to tie down the end to keep it horizontal for a while, anchor the base of trunk to opposite side to avoid putting stress on the roots. it'll cause resistance as it tries to grow upward, which can make it lurch in the soil and potentially tear some finer roots.
Just bending branches outward is good enough for autoflowers. This LST might get you a slightly lower portion of larf at bottom, but overall they yield will be what it is, regardless. They are typicalyl small palnts. you don't need to do much but spread them out as they grow. Since you can only guess as to how much vege time you have, it's also more difficult to level everything out.. if you guess wrong the apical meristem (the 'trunk') will either surpass the rest or not catch up...