1. Explain it simply
Put 别 (bié) or 不要 (bùyào) right before the verb to make a negative command.
别 and 不要 mean the same thing here; 别 is shorter and more common in speech.
Add 了 at the very end when you want someone to stop an action they are already doing.
2. Show examples
别看电视了。
Bié kàn diànshì le.
Stop watching TV.
别 + verb + 了
不要玩手机了。
Bùyào wán shǒujī le.
Sandhi:Búyào wán shǒujī le.
Stop playing on your phone.
不要 = 别
别找了，手机在桌子上。
Bié zhǎo le, shǒujī zài zhuōzi shàng.
Stop looking, your phone is on the desk.
stop the action
3. Share a tip
Keep the order fixed: 别/不要 first, then the verb, and 了 last. Both 别看电视 and 别看电视了 are correct, but never put 别 or 了 after the verb.