It’s often said that the only way to really learn something is to teach it; when it comes to GMAT quant problems, you could say the only way to really learn something is to create it.
To get started writing your own variations on GMAT questions, take a problem (preferably from The Official Guide) that you’re not 100% comfortable with. Then start asking yourself “what if?” What if x was negative? What if there were four coins instead of three? What if you added two or subtracted two from one of the numbers?
How you change the question will differ in every single example. It’s a test of imagination nearly as much as a test of knowledge. But if you generate, say, 10 new GMAT questions, you’re much closer to being inside the head of the test-maker. By extension, you’re that much more prepared for what the GMAT will throw at you when you take the real thing.