In order to unlock the "Sketchy" trophy in Life is Strange 2, you must start any drawing in the episode by interacting with a drawing spot.
Without spoiling anything, the first spot you'll find is by the river in the woods, but it will only appear after you have gathered wood for a fire (but BEFORE you ignite that fire). Head to where Daniel is trying to fish after setting the logs up and a rock just behind him will have a prompt to start drawing.
To draw, look at the view and hold [cn_x] to make an observation where the dotted lines overlay the view. Then look down at the sketch pad with the right analog stick and start waggling the left stick to fill out a drawing. A full sketch will take several observations and sketches to complete, but you can Leave any time after the first sketch to unlock the trophy.
Here's a screenshot of the location from which you can make a drawing: