For the first 3 weeks eat normally. Record everything that goes into your mouth. If you have a kitchen scale that's the best, if not estimate as best as you can (you get better at it with practice). Using the weight of the food consumed, look up the calories on the internet. I keep a file on my smartphone. Every time I eat, I write down the estimated weight, what it is (don't forget drinks, most people can lose weight simply by cutting out high calorie drinks) and how many calories are in it. Total your calories daily. Find out what your average calorie/day intake is over the 3 week trial period (total daily calories/21 days). This is your starting point.
Every Monday morning, step on the scale and record your weight after taking your morning piss. Don't step on the scale at all during the rest of the week. Record the weight
If you want to lose weight, subtract 100 calories from your 3 week average calories/day. Make sure you don't eat more calories than this per day. Check the scale after 1 week. If your weight went down continue at this level. If your weight went up or stayed the same, subtract another 100 calories per day.
Continue until you reach your goal.
So for instance if your average calories/day during the trial period was 3500 calories/day, you would start by eating no more than 3400 calories/day for the first week. If your weight stayed the same, you would drop to 3300 calories/day. If your weight dropped, you would continue eating 3400 calories/day for the next week.
I've used this technique to cut and bulk as needed over the years. I've gone from 245 down to 195, back up to 255 and down to a very fit 230 (I'm 6'5"). I bulk in the fall/winter to maximize strength and I usually cut 10% of my body weight in the spring to look good.
The reason it works is that you make changes slowly (+/- 100 calories/week). You don't shock your system or mess up your hormone balance. I can comfortably change my weight to whatever I want it to be and honestly it doesn't feel like a chore if you do it slowly. Getting used to dropping 1000 calories/day slowly over time is easy. Suddenly dropping 1000 calories/day from one week to the next is not sustainable.
I find a lot of comfort in that it is just a math equation, just be honest about what you record calorie wise.