To be in deficit, you need to consume less calories than your baseline.
To find your baseline you need to find out how many calories you need to consume to not lose or gain weight. So treat it as an experiment. The question is how many calories is your TDEE.
The experiment would be eat X calories over Y days, check weight change over Y days. The less weight change, the closer X is to your TDEE. The next part is eat Z calories over Y days and check the weight change over Y days. Evaluate.
Note your TDEE changes based on your daily routine and activity level so it’s never set in stone.
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And then to lose weight you just subtract how ever many calories you can from your TDEE. The google says there’s 3500 calories in a pound of fat. Cutting 500 calories from TDEE per day can make you lose 1 pound of fat per week. This is super simplified but keeping it simple might make it ‘stick’ better.
Everyone is different. Assume 2000 calories. Aim for 2000 calories or a 3 day average of 2000 calories and keep track of your weight. This needs to be measured at the same time every day. When I was hardcore on IF it was early in the morning, right after washroom & showering but before getting dressed. Birthday suit, on the scale, off, get dressed. I don’t care about the evening number… just that morning one.
40’s Male, physical job, targeted 1600 calories and was able to drop 2-4lbs a week. I was hungry at times and my cheat snack was lightly spiced & grilled chicken breast I’d already sliced into “fingers” and kept in the fridge in tupperware. 3 of those would give my stomach something to work on. I did not include those in my daily intake numbers and yeah they were outside my window. I did not take them every night. Just when I was definitely hungry but it wasn’t sleeping time yet.