Very carefully.
I think you're going to find that this is one of those times were artwork just doesn't work in real life. As I recall, Spiral's always been drawn with a fairly normal looking torso. Unfortunately, there's just no room for three arms, on the standard female shoulder. Spiral seems to have some kid of a dimensional rift going on, that allows her shoulders to acuupy the same point in space.
The only thing I could think of, would be to move the front pair of arms forward a little (too far, and it'll just look weird) put the second pair imediately behind those, and the third pair on her back. The real problem is gonna be internal space. You may have to adapt the shoulders to use narrow pins, similar to what elbow joints use. It's either that, or use ball joints, from a Legends class Transformer, but I don't know if those could be made to look right, on this kind of figure.
Good luck! If you pull this off, I really want to see it. I always thought she was one of Marvel's cooler looking characters.
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