Get a traditional trouser pattern and modify it. By "traditional", I mean, just make sure it has a high enough waist.
The fishtail can be cut with the body fabric or as an addition. The addition basically has the body fabric tapering in at the centre-back seam and tapering out/going straight with the additional piece. But this varies with the body. The best way to do it is to make a linen mock-up, cut that piece off, adjust to fit, and add it back on.
A pattern for classic jeans may actually be shaped this way.
And here's an actual pattern :
http://www.reconstructinghistory.com/products/rh919-1830s-1900-trousers-1Also, you can just draw your own pattern from scratch :
http://www.costumes.org/HISTORY/100pages/1893to1898cuttersguide.htm (Part 3)