Setup
Everything begins with lighting. Look for diffused natural light, ideally coming from the side. If indoors, this typically comes by setting up about 0.5 to 1 meter from a window. If outdoors, find a space with even light, avoiding hotspots and shadows. Often this means looking for a spot with consistent shade.
The next component is the background, which is as simple as finding a neutral background (grays and blues work great) with minimal pattern. Avoid brick, stripes, dots, and other distracting patterns. The subject should really pop, and this combination of lighting and background does the trick.
Framing
For still lifes, framing the shot is key. While you can crop later, getting a correctly framed shot from the right angle and perspective is critical for ensuring that you have the best photo to work with. With iPhone, I nearly always choose the lens with the closest focal distance. On iPhone 12 Pro models, this is the Telephoto camera.
When framing a floral arrangement, place the subject in the center and make sure that the frame is evenly filled. I tend to handhold cameras, including iPhone, and use a slight downward angle (just a few degrees) from head-on with my subject. This ensures that you can see the vase, but also get great depth and dimensionality from the flowers, which are the stars.
The Telephoto camera provides 2x optical zoom range on iPhone 12 Pro and 2.5x on iPhone 12 Pro Max.