Filling the Page

I’m taking a sketching class. I feel like a beginning writer does. Do I know what I’m doing? Is any of this any good? What am I doing this for, I can’t draw/write?
The mandate was to just draw what I saw, to narrow it down. To just pick a small section of something and notice the lines and the spaces in-between.
Writing is like that too. You narrow your vision and write what is in front of you. You take that small incident of memory, if you’re writing memoir, and paint the picture you see in your mind’s eye with words. You don’t write the whole book, you just write the piece you ‘see’.
So, in sketching, I’m advised to just pick a part of something. Not the whole wall of mixed-colored stones, and the trees and shrubs rising out of it, and the green railing above it, and the cloudless blue sky beyond all that, all of it running for miles, with a train tunnel interrupting it at one point. So, I just pick a section of the stone wall and begin to notice more than smudges gray stones.
I see crevices, pink stone blending into gray, and one rock places at an angle that is like a burnt sienna crayon.
If I stay with what I see, it fills the page, like the blank page in my writing notebook begs to be filled.
I take out my colored pencils, markers and pens, and begin. My aim is to fill the page.
Jyoti