It's best to develop a good flute embouchure from the very beginning.
Embouchure is a French word for the position of the lips and muscles around the mouth. The primary objective with the flute embouchure is to keep it relaxed. A tight, pinched embouchure results in a tight, pinched tone. Learning how to play the flute is much easier with a relaxed embouchure.
Don't develop the tight, "smily" embouchure, or you will find it a difficult habit to break.
A tight embouchure also leads to bad intonation. The high register, in particular, will be extremely sharp compared to the lower octaves. The low octave will tend to be flat in comparison. In short, you will be out of tune with yourself!
Try blowing across the top of a pop bottle. That's a glass pop bottle, not a plastic one. These days it's harder to get a glass pop bottle, but it's a great way to demonstrate the loose, relaxed embouchure. If you pinch and pulled back the corners of your mouth, it is very difficult to get a sound when you blow. Now relax and blow.
Blow through your lips like you are trying to make the sound of a horse. Your lips must be loose and relaxed, not stretched.
Practice in front of a mirror so you can see what you are doing.
Certain "extended techniques," such as whistle tones, are helpful and fun for developing a relaxed embouchure.
Surprisingly, flutists can use some of the techniques employed by brass players for developing a good embouchure with related benefits of good tone and efficient air usage.
We might not think that the flutist's embouchure has much in common with the brass embouchure, but the buzzing technique they employ can be very beneficial to flutists, too.