Then most artists make a square with their fingers as a viewfinder to figure out whether they are going to place the canvas in portrait mode or landscape mode and what should be in the composition. There are commercial viewfinders but one made with your fingers is much more versatile. The problem is you have to know how to do it.
One done like this with the same fingers on both hands touching is WRONG. It is hard to make it square and it is hard to enlarge it and change its dimensions. And your fingers get into the picture.
Now you can have lots of fun moving the finger viewfinder around. In fact that they whole point of it. It's not that you are going to make a likeness of what you see through the viewfinder. You are going compose a picture, see what to leave in, and since you can see outside of the view between your fingers, you are going to see if there's something outside that you are going to move inside.
Picture composition is a wonderful subject and sometimes it's fun to spend a lot of time playing with this viewfinder. Nature is not particularly adept at composition, and it is usually a mistake to try to copy exactly what is out in front of you. Nobody is going to know what there. They are going to judge what you make of it by aesthetic criteria.
Why does the framing of something help, in this case with your fingers? The best way to illustrate that it does is to find a newspaper and take a random part of a paragraph and draw a frame around it with a pencil. Then look at the whole page. Your attention is drawn to what in the frame. And unless you intend to paint on the wall which some artists do, you are going to look at this painting in a frame. In fact, I would suggest that you draw a frame on whatever support you are going to do.