There are plenty of posts for rules on numbers, but I can't seem to figure out how one would say, for example, -5 (negative five). I know that negative number is 负数, so would it just be 负五？
Negative numbers are simply read
负xxx, for example:
(-3) x 5 = -15 负三乘以五等于负十五
When applied to certain domain, there might be domain specific way to read it. For example for temperature, it's more common to read it as 零下 (literally 'below zero'):
负十度 is also correct, just less common.