The entire ghazal has to be in one particular 'beher' [among many] & all couplets in the ghazal will be in accordance with this pattern. For ages, 'faaylun, faaylaat' has confused young writers.
To some, it comes naturally. You remember 'dohas'--the laghu [1] & guru [2] or even in lyrics [movie songs], the 'dhun'. Classical masters, bards who never studied, knew it well, it came naturally to them. Others learn it.
Initially, it appears too tough & master poets tell you basics, correct the mistakes. If one knows the system, it's good. If you don't know, try to learn. In many languages, poetry is no longer dependent on prosody. But it is also true that this art is the reason Urdu couplets and ghazals have tremendous reach, quotability and are remembered.
Don't dissuade: Knowledge of Prosody is not for patronizing, rejecting
Everyone is free to write. And, knowledge of prosody must not be used to deter and dissuade anyone. As the 'ustaad-shagird' system has weakened in recent decades, unfortunately, some senior poets, don't help, rather their command over the subject becomes a ruse to reject others.
I remember, I was sitting with veteran poet, Kausar Siddiqui, when he was asked by a young boy about 'arooz'--prosody. He instantly took out a piece of paper and started explaining the system of 'beher'. There are ways to easily make you understand when a teacher is around. How the words are broken, where you can take liberty, how much is allowed, etc.
There are arguments in favour and against this system. Debate has been going on for decades.
For example, a youngster has a superb potential, he has imagination. He writes a ghazal or nazm, the master poet says, 'it is kharij az bahar' or simply junk, this is patronizing. Rather than telling, correcting and asking him to keep writing and learning, he tries to impress him with his knowledge of a 'lesser known art'.
Uniqueness of Urdu poetry: A tradition of centuries and focus on art, language
It's not that 'arooz' is something that should scare us. It's an asset. When it comes to Urdu poetry, it is the uniqueness. And, Urdu has an extremely tough system about judging poetry because of a long history, tradition, three hundred years of master poets who took inspiration from Bedil and Hafez etc.
You must not patronize, decide or control what others write
Most basic thing in life is that, one must try to learn and if someone asks you something, share your knowledge, help them out. Lekin aap izhaar par 'qaid' bhi nahi laga sakte. If someone writes, he-she is satisfied, adhering to meter or not, it's fine.
They seek your help, you should give them the necessary tips. Every ghazal or nazm may not be literary as per your standards. But everyone has the freedom to express themselves. You can't hold others at gunpoint--write in meter or I'll not consider you a poet unless you prove yourself in this format.