Equation writing is not a one post or one blog process. It will not come all of a sudden from zero to maximum. It requires a practice of many days or even months for who are not used to mathematics.
So start building equations, first of all read elementary books in mathematics (Algebra, Arithmetic, and Geometry). Since we are talking about GMAT so do not read topics outside of it. Try to solve all the exercises in the book. Once you are done with this, gradually start using GMAT books in the order (Priceton, Kaplan
, and Manhattan). Together all of these should give you firm understanding of GMAT mathematics. Finally take the test from GMAT club and you will see your score zooming.
One more advise, do not always try to zip through answers. Check why your right answer is right and wrong one is wrong. I mean spend a good deals in understanding the explanations.