There are three methods to construct confidence intervals for ratio parameters, namely: (1) Delta method (2) Fieller’s method and (3) profile-likelihood based interval on generalized linear model (GLM) technique.

Here is a very good note on Taylor Approximation and the Delta Method.

Another one

Refer to: 1.Beyene, Joseph, and Rahim Moineddin. “Methods for confidence interval estimation of a ratio parameter with application to location quotients.” BMC medical research methodology 5 (2005): 1-7.

2.http://www.biostat.umn.edu/~chap/S08-EstimateRatios.pdf

3.https://en.wikipedia.org/wiki/Fieller%27s_theorem