How do you get a software patent?

By filing a patent application with a well-drafted specification and prosecuting same to grant!
Naturally, the details depend on the requirements of the national or regional patent office examining the patent application.
If you may consider to file - later on - a patent application in Europe, for example a European patent application to be examined at the European Patent Office (EPO), the specification should already be drafted under due consideration of applicable requirements!
In Europe, patents are generally granted for any inventions in all fileds of Technology, provided that they are novel, involve an inventive step, are suspectable of industrial application und are regarded as inventions. Among other things, business methods and Software AS SUCH are not regarded as inventions.
So, in principle, computer-implemented inventions (CII), i. e. inventions implemented in software, hardware or a combination thereof, are patentable inventions.
Having said that, the main reasons for refusing CII patent applications - after examination of the TECHNICAL features - are lack of novelty or lack of inventive step.