If the canvas you are stitching has a shape drawn on it--say a rectangle, square or round, you can just go with the artist's shape. For the Christmas tree canvas I am working on I need to "draw" my own shape. I don't want all that blank space at the top of the tree. I started with drawing straight vertical lines on either side of the tree, about halfway up. This shape is made more challenging because the star at the top is not centered.
I couldn't find my new favorite for drawing on canvas--a ceramic pencil that will not rub off on thread but can be erased! So I drew the straight lines with my second favorite canvas marker, the micron pen. It is acid free and is suitable for archival pieces. Unfortunately, mistakes must be painted out.
To shape the edges above the straight lines, I took random scraps of Vineyard Silk Classic and tacked them in places. The resulting shape can be manipulated as you go. And of course, I am counting on my finisher to make the final shape choice.