How did the conversion of celsius to Fahrenheit come about?

If you mean the formula, both scales are linear with two known points of 32^oF = 0^oC and 212^oF = 100^oC; plotting Celsius on the X-axis and Fahrenheit on the Y-axis, it is a simple matter of finding the equation of the graph F = mC + y:

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The Celsius scale was devised to be freezing point water = 0^o, boiling point water = 100^o.

I think the Fahrenheit scale was devised to be freezing point salt water = 0^o, normal human body temperature = 100^o; however the early thermometers were not very accurate hence normal body temperature is about 98.6^oF. Salt water freezes at a lower temperature than pure water, hence freezing point of water = 32^oF.