As for when to check your blood pressure, the most important thing is to do it consistently the same time of the day (ask the doctor which time he prefers and also what time in relation to taking your medication). The following article has a lot of good information for someone just starting to monitor their blood pressure: https://www.drugs.com/cg/how-to-take-a-blood-pressure.html
"Blood pressure guidelines are not updated at regular intervals. Instead, they are changed when sufficient new evidence suggests the old ones weren't accurate or relevant anymore," says Dr. Paul Conlin, an endocrinologist with Harvard-affiliated VA Boston Healthcare System and Brigham and Women's Hospital. "The goal now with the new guidelines is to help people address high blood pressure — and the problems that may accompany it like heart attack and stroke — much earlier."
Imagine a garden hose hooked up to a spigot. When the hose is flexible and there are no kinks in it, you can turn on the water full blast and it will flow easily through the hose. But if there’s a kink in the hose, the water doesn’t flow as well beyond the kink. And the pressure inside the hose builds up behind the kink. Or imagine there is gunk inside the hose blocking the path of the water. Your arteries are a lot like that garden hose.
If you had previously been diagnosed with high blood pressure, the new guidelines don't affect you too much, says Dr. Conlin, as you still need to continue your efforts to lower it through medication, diet, exercise, and weight loss. "However, based on new information in the guidelines, your doctor may propose treating your blood pressure to a lower level," he says.
Blood pressure readings fall into four general categories, ranging from normal to stage 2 high blood pressure (hypertension). The level of your blood pressure determines what kind of treatment you may need. To get an accurate blood pressure measurement, your doctor should evaluate your readings based on the average of two or more blood pressure readings at three or more office visits.