They will be using all of the paper. Ask them to hold their colored paper vertically for a vertical picture, or horizontally for a horizontal picture. Starting at the bottom of the piece, demonstrate how to cut an interesting abstract shape, eventually returning to the bottom of the piece. Have students glue one of their negative shapes in a top corner of their background paper, lining up the edges neatly. Repeat the process on the other half of the background paper, reversing the location of positive and negative.
Students are now free to add other shapes, particularly ones which repeat.
Students may wish to find recognizable objects within their pictures and continue along that theme, or they may wish to keep it nonobjective. Towards the end of this project, I allow students to use my paper-scrap box for additional pieces, and crayons for lines.