Use it to test 2 hours after the main meal of the day. Keep a record of what the meal consisted of. keep a record of the reading you get and yes I know the meter has a memory but it is helpful to see the meal and the reading together.
The object of the exercise is to alter the meal to get a reading of less than 8.5. You may not have to alter the menu much but I predict that you could be eating less flour, rice or potato in the future but then again it may be that one of them does not cause you trouble.
Don't go too mad. Just concentrate on one meal of the day for now until you get the hang of it.