This is exactly what I have been doing for several years now. My goal has been to read many of the classics of math (Gauss, Moebius, Euler, Courant, Hilbert, etc.) in German (or French). Noting (!) of course that Gauss and Euler wrote in Latin .. older translations were into German or French (similarly for example for writers like Galois, or Abel, which I have been able to find in German only).
Here are some of my suggestions.
I like this dictionary: German-English Science Dictionary (Louis DeVries and Leon Jacoley). It's a lot faster to look up mathematical vocabulary in it than a more complete dictionary (of course you will also need that as a last resort).
Start with something that is either easier (an introductory text), or that is harder but is something you are very familiar with, so in either case you recognize the topics and in some cases the theorems and the development are immediately recognizable to you.
In the case of some of the more famous works (Disquisitiones for example), you can also obtain an English translation to use when you get stuck (amazingly, Disquisitiones did not appear in English until the 1960's!). In my area of interest I have also found both German and English versions of Hilbert, Courant, and others.
I have been pleasantly surprised overall at how much easier it is to actually read German when limiting myself to math. I would have difficulty carrying on a conversation in German, or reading a newspaper, but I have for example read most of Moebius' Astronomy book (an introduction to the subject), and pieces of several other works including Disquisitiones, and feel that I have been able to comprehend these just fine, albeit slowly at times.
It's a fun project! Just think: In some cases you are understanding a book written in German better than a native speaker would because you understand the math, even though you would (well, I don't know about you, but I know I would) get lost in a supermarket. Good luck with it!