That white plastic thing is what does the stirring. Just drop it in the vial of solution, and then turn on the device. Rinse it off well between types of tests. Some tests may have you use the same sample and put different number of drops of more than one reagent - it can stay in the vial for those. But rinse it off (and rinse well the vial) if emptying the vial to use for a different test.
In Pool Math, if you enter the value for CYA, it will change the recommended CL target you should be at.
CYA is the hardest test to judge the value to use. Don't stare at the dot, only glance at it - add a bit more, and glance again. Round up your reading to the next highest line. Try to be in a place with bright but even lighting. Outside with your back to the sun works well. It may take a few times before you get the hang of it and get consistent readings with the same sample of pool water.