Try sketching a loose arrangement of blooms - just laid in a heap - rather than an artificial formal arrangement. Don't obsess over the outline of each petal, but rather try to suggest the overall shapes and textures. Use hatching and scumbling to add detail and texture in the shadows. Using a variety of flowers, some with more defined shapes such as tulips or lilies, and some with looser forms, such as David Austin roses, helps to add structure and interest.
An informal composition like this can work well when combined with a crisp geometric element, such as a precisely drawn frame or pattern. Try adding variation by making one or two blooms slightly more exact and some looser and more abstract. Use waterproof ink and add a coloured wash across the blooms. If using digital media, this can work well layered over a manipulated photograph to combine elements of realism and abstraction.