>>7626844 (OP)You have to start with the soft hourglass-y form of a woman. Women don't really have the same kind of broad shoulders and muscle mass that men have so the shape of a trans man has this permanent kind of feminine feature to it.
You could maybe sag the distributed fat a little near the armpits too. A lot of FTMs have that bulging side flab on the flanks of their torso.
Once you have the basic forms down you can add in details like maybe the neckline or the scarring itself. Be careful not to make the pectorals too large, women don't really have the mass so it mostly comes off as soft and squishy or underdeveloped, like if you were painting a boy before puberty.
The scar line is like a jagged ring around the form of the torso. It's less of a parallel line and more like a slightly curved one, to accommodate that female form underneath.
Optionally you can add in some weird looking nipples, stick them wherever you like, like the surgeon does lol.