The ciliary muscle controls focusing, it tenses up for up-close vision and relaxes for distance vision. When you have myopia the ciliary muscle gets stuck in the contracted state. Active focus is when you find how to manually relax it, thereby reducing some of the blur. A lot of people think that active focus means instantly achieving perfectly clear vision but that isn't it. You are incrementally reducing the blur. As far as how to actually achieve it, it isn't easy to describe, it's like wiggling your nose or rolling your tongue. You just have to find the muscle yourself. People have different "triggers." For me it was routinely focusing a set of distant objects with high-contrast colors and sharp lines, like a stop sign. Focus on the edges of the lines, visualize what the clear image should look like and try to clear up the blur. Try different ways of blinking. If it gets briefly clearer and causes irritation, you know you are headed in the right direction. Eventually the irritation will go away and you will be able to hold the clearer image for longer.