The sentence is:
MC is revealing a secret how to win every dice game. But I'm not sure the meaning of his words. Does it mean throwing dice one by one?
ひとつ（で） actually applies to the previous word 投げ方 here. It's kind of like だけ in this usage; he is saying to become able to control the result of the dice solely through just the 投げ方 (way of throwing).
In the above sentence, the 投げ方 is described as the sole deciding factor for producing the result.
Wiktionary has a good example of the usage:
That's not to say that there aren't other ways the situation could be resolved, but a reply would serve on its own to decide things all by itself.
大辞林 also highlights its use in this kind of situation.
大辞泉 is more plain about it and says not much more than だけ, but I get the impression that it's stronger than だけ and gives a greater sense of exclusion.
(translations are mine)