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
I just started using high blood pressure medication and purchased a blood pressure meter at age 72. what concerns me is that the BP readings I get have a very wide variance in. I measure every morning as soon as I wake up, while still in bed and prior to having any coffee. I take 3 or 4 readings. I have found that on any given day my reading can vary from 126 to 143 systolic and the diastolic reading from 68 to 87. this wide variance makes it very difficult to track trends. My questions are: is this normal? is my instrument possibly defective? How can my cardiac doctor make a decision to put me on life long medication based on SINGLE reading in his office?