I feel like how to start a calorie deficit is an extremely common question that there's not actual post on but just random answers on other posts here and there. I see myself answering questions on it time after time because I have successfully achieved this myself, so I think it's necessary to solve this question with a post dedicated to this topic exclusively and I want to explain how I did it and how you can achieve it.
Knowledge is power, so in comments you can find scientific researches and resources that can help you achieve this in a healthy way.

⚠️DISCLAIMER⚠️
This post is based on personal research on the topic. Please do your own research too before attempting this AND consult a health professional or nutritionist. Even though this information is valuable they are the only ones that can give personalized advice and feedback.
Do not attempt a calorie deficit if:
- Under 18 yo (only under explicit medical recommendation and guidance)
- Negative or toxic relationship with food or calories or other mental health issues (consult a mental health professional and nutritionist instead)
- Not overweight (attempt recompositioning instead)

The main important thing to understand is TDEE.
Your TDEE is your Total Daily Energy Expenditure = the calories you burn just by living. This is UNIQUE to every person because it depends on individual biometric factors that will vary depending on the person.
For this reason, you should not follow online advice on how many calories you should eat. This should be specifically targeted to YOUR individual needs.
You can use a TDEE calculator: https://tdeecalculator.net/

Your body will generally lose 0.5kgs per 500kcals deficit (per week). This will lead to healthy and sustainable weightloss (-0.5~1kgs per week). This is because ✨calories are just energy✨ and while on a kcal deficit your body will use its own resources to produce energy (aka, it'll burn fat to produce energy).
To achieve this, I calculated how many kcals we must eat to achieve that deficit. This will be your calorie budget.
How do you calculate it? easy:
➡️ TDEE - DESIRED DEFICIT = CALORIE BUDGET

After learning my own calorie budget, I realized I needed to track my calories to make this work out.
As we said before, calories=energy, so you should not feel bad about eating a bit more than your budget on some days. This will not affect long term progress.
I recommend chronometer for this.

⚠️IMPORTANT: Make sure to NEVER neglect your nutritional needs. ALL nutrients are necessary, and if you feel like you can’t achieve this on your own, PLEASE seek a nutritionist help because it’s so worth it.

⚠️Exercising will help you achieve your kcal deficit much easily. It is necessary for good long-term results!! Even if it's the lowest % of kcals burnt daily you still will need to exercise to achieve weightloss. So don’t be skipping your workouts and stick to the programs!!