Get a math book that teaches the fundamental rules of math. "Math for Dummies" or "Math for Morons Like Us" are good books to start with because they cover a wide range of math concepts.
Review and relearn concepts such as addition, subtraction, multiplication, integers, division and algebra.
Build on your learning by doing practice problems. Once you understand a concept, practice problems solidify that knowledge so that you can build on it.
Test yourself regularly. Make sure that you are moving at a quick pace through the concepts, then you can continue to build on the knowledge you gain.
Make it fun. Use snacks and candy to emphasize addition, subtraction, multiplication and division concepts. Use rewards as motivation.