Go to a bookstore and purchase a basic math textbook. Do not go beyond geometry and algebra. Some good books are Basic Essentials of Mathematics, Book 1 by James T. Shea, Master Math: Basic Math and Pre-Algebra by Debra Anne Ross, or Basic Math Skills by Ags Secondary.
Practice the basics--addition, subtraction, multiplication and division. This is where the flashcards come in handy. Use the flashcards to teach your brain to rapidly add, subtract, multiply and divide. You can at first use paper and pencil to work out the problems, but eventually you should be able to do them in your head without the pen and paper. Try not to use the calculator for this basic math. If you need, only use it to check your answer is correct. You should be your own calculator at this point.
Ask someone to help you with your flashcards. See if you can do them from memory. See if you can recite the square of one, the square of two, the square of three, and so on. You should be able to recite up to 20. If you cannot recite them, try making your own flashcards of these. Making them and then using them will be another layer of repetition and helping you master it.
Turn to the math book and start reviewing the next level--fractions and geometry. Learn the terminology first and then concepts and formulas. Take it slowly and be sure to master each concept before moving on. Math builds on itself, so if you don't understand one thing, it will affect your understanding of a more complex concept later. Proceed chapter by chapter making sure you understand everything in the chapter before you move on to the next chapter.
Continue repeating the flashcards daily. Continue to recite the squares of 1 to 20. Once you become comfortable with them, graduate to doing them once a week.