Using an HR monitor is a great way to make sure you are working hard enough and long enough. You have to burn off 3500 calories to lose one pound of fat, that is where the math comes in of burning 500 calories every day for 7 days. If you work really hard (sweating & swearing) you will burn off more weight because it will increase your metabolism for days after the strenuous workout (not dying, not trying). You have to burn them off in 45 minutes or less, the goal is 30 minutes. The weight won't come off nearly as quick if it takes you longer to burn off 500 calories. Either way, don't stop training until you burn off 500 calories or more.
This hard workout will only work if you log everything you eat into the "free" my fitness pal app. Enter the food before you eat it, this helps keep junk food out of your mouth. Try to keep your carbs close to 75g a day, Keto people are under 50g a day. You have to watch your carbs and calories because no matter how hard you work out, you can't work off a bad diet. Calorie totals are specific to gender, age and activity level, I would have to consult with you to determine this for each individual.
The picture of HR monitor is a Wahoo TICKR X Heart Rate Monitor with Memory, Bluetooth / ANT+. I included the link below, It was rated the best HR monitor in 2019. A chest strap monitor is the most accurate, but if you already have a Apple watch or Fitbit HR, just use that, no need to spend more money.
Let me know if you have questions or need help.