How to write a law?

The key thing with a law to make it watertight is defining exactly what you want to ban. What are saggy pants? What counts as being saggy? You've got to find a definition that no lawyer can argue their way around and that cannot be misinterpreted.

And do you want to ban this only in public, or in private as well? You'd certainly create trouble if you banned it in private, where after all, people can be naked if they want. So what's a public place, or does the law already define what is public?

What are pants? How does this differ from, say, the bottom half of a bikini?

Once you have a definition, you've done the hard part. Now it's just a matter of saying it's a felony or a misdemeanor of whatever class you want it to be to wear whatever you have defined as saggy pants. And if the law of wherever you are writing this for does not prescribe the punishment for this kind of felony or misdemeanor, you'll have to say what the maximum sentence should be.

Your teacher should outline your assignment, even if it's brief. If not, then you will have to outline it yourself.

First of all, think about the reason why people advocate such laws. There are already laws on the books against indecency. Why write a redundant law?

No physical harm comes to a person when others wear saggy pants.

No genitals are showing, so there is really none of the aforementioned indecency.

Any merchant can invoke his dress code if such a saggy pants person comes in the store, so no law is needed for that.

The only purpose and use of your assignment would be to recognize how such laws are pointless. They waste legislative time, and hence, tax dollars. Such laws reveal the sheer pettiness of people.

That would depend on which level of government the law is intended for. I'm guessing this is some kind of homework assignment for you, but without knowing the details of the assignment I can't give you a good answer. My only suggestion is to go online and read some actual laws to get a feel for the language and structure typically used.

As your choice of clothing is an expression of your freedom, you will need to come up with a compelling state interest to outlaw saggy pants.

Find a compelling state interest, and fashion your law around that.

Maybe a safety issue? Dumbasses who wear saggy pants might trip more?

You have to define exactly what a saggy pants is, where saggy pants would be legal and not legal, and what penalties would apply to violators.