Easy right? Just don’t eat or drink too much? SOOOOOO not easy! Haha :D How do you even tell if you’re eating or drinking too much? You’re gonna be surprised at how big serving sizes are actually supposed to be (I sure was!) For example, a single serving of:
Vegetables or fruit is about the size of your fist.
Pasta is about the size of one scoop of ice cream.
Meat, fish, or poultry is the size of a deck of cards or the size of your palm (minus the fingers).
Snacks such as pretzels and chips is about the size of a cupped handful.
Apple is the size of a baseball.
Potato is the size of a computer mouse.
Pancake is the size of a CD.
Steamed rice is the size of a cupcake wrapper.
Cheese is the size of a pair of dice or the size of your whole thumb (from the tip to the base).

Whaaaaa? That’s like.. no food! I’ma die of starvation!
Dude, you can have more than one serve of stuff it’s ok! Check out this table below for the recommended serves.
Like I’m always harping on about, it’s not something you should rush into. And also you need to be eating CLEAN to eat less. You need to be getting nutrients in with every meal. If you’re not eating big meals and they’re made up entirely of saturated fats and simple carbs and sugars, you’re going to have a bad time, and become malnourished! Notice this table doesn’t include food that has absolutely no nutritional value? :P
How do you even go about doing this portion control thingo?
Some tips are - try using smaller plates! Serve the right portions and don’t get seconds, freeze your leftovers straight away in correct portion sizes, and don’t keep platters of food out so you’re not tempted to pick!
If you’re eating out, ask for smaller portions, or share (like Ant and I do!). You can sort out what a good portion size is for you and then ask them to doggie bag the rest.
When you’re buying things, try not to get small sized snacks like chips and crackers and things you can just nom on. It’s really easy to eat too many. Try and get individual servings of things.

Reference
Mayo Clinic: "Energy density and weight loss: Feel full on fewer calories."
MedlinePlus: "Health Tip: Control Portion Sizes at Home."
National Institute of Diabetes and Digestive and Kidney Diseases: "Just Enough for You: About Food Portions."
Academy of Nutrition and Dietetics: "Ways to Shave Calories."
https://www.eatforhealth.gov.au/