The phrase "a mental note of what" is correct and usable in written English.
It can be used when referring to the act of remembering or keeping track of something in your mind for future reference.
Example: "I made a mental note of what she said during the meeting so I could follow up later."
Alternatives: "remember what" or "keep in mind what".
She learned to combine flavours and textures by making a mental note of what ingredients go well together.
I tried to make a mental note of what he had done, but it was no use: Chef von Bargen was simply too quick and too practiced.
Lets make a mental note of what not to ask Charlie Sheen at a dinner party.
If I'm being restrained, I'm making a mental note of what I'm being restrained with, as well as passively resisting the restraints (flexing my wrist muscles, presenting my hands to be handcuffed in front, separating my wrists, etc).
Sometimes the answer doesn't come as easily as we'd like and it's left up to trial and error - taking a mental note of what's helping you and what makes your symptoms worse is a great idea.
Make a mental note of what you can smell.
And equally unaware that his first message would someday be the object of historical scrutiny, Mr. Tomlinson said he made no mental note of what he first tapped out on the keyboard.
If you don't take mental note of what the other person said, it's going to be hard to use those conversational nuggets as reference points when you're asking questions.
"It's the kids versus the kids, so I don't even notice it". What Glenn made a mental note of was how his Sailors outplayed the Breakers late in every set.
If you have passengers eating with you, ask them what they want and make a mental note of it.
When I'm out of the house, I struggle to pick my husband and children out at all – unless I can remember exactly what they are wearing – so I always make a mental note of it as we go out.
