Every physical activity our body does has to be mentally created first. If you wish to lift a glass full of water, first your brain has to instruct your hand to do the act. You mentally do the act first and then do it physically.
When you decide to write something, you start thinking of that something. If you are writing about the discussion you had with your boss then you are mentally recollecting the past discussion. If you are writing about how you will be enjoying your vacation from tomorrow you are imagining the future.
So that is a conscious mental activity that you are doing while writing.
You are also mentally selecting words, framing sentences and spelling each word while writing it.
These mental activities are also conscious activities.
The formations of the writing, the space between lines, words and letters, the size of the handwriting, the slant of the writing, the pressure of the handwriting, the margins on all sides of the paper etc is being taken care of by you.