note down
Separable
Write something briefly so that you remember it
Let me note down your phone number.
Separable
The object can go between the verb and the particle: turn the light off, or turn off the light. When the object is a pronoun it must go in the middle: turn it off, never turn off it.
The particle: down
This verb belongs to the group ‘writing it down’.
Others in the group: write down, jot down.
Practise it
Add it to your daily practice or see every verb with down.
Further reading: note down on Wiktionary (CC BY-SA).