If you're asking about a specific doctrine like the deity of Christ or the transubstantiation of communion, then let us know.

If you're asking about the whole body of doctrine that the Roman Catholic Church upholds, then the answer is that it was a very gradual process. One way to view the development of the modern-day doctrine is through the long series of "church controversies": or in other words, very heated differences in interpretation of scripture or tradition. These controversies often led to formal councils being called. The most famous "ecumenical councils" are found here.

All seven of those ecumenical councils involved representatives from many, many regions that all had major Christian churches. Debates ensued, and formal written creeds usually followed. Please note that all seven of the ones I linked to above happened before the East-West schism, so there were no established denominations or different churches like we think of them today. After the schism and before the Protestant Reformation, Thomas Aquinas was the most influential individual in the development and refinement of Roman Catholic doctrine. Over time, the church grew to fully accept and affirm the ideas he wrote about in his magnum opus, Summa Theologica.

As far as post-Reformation, I'm not knowledgeable enough to even give a summary of how doctrine has developed. I'll defer to others for that information.