Start with a useful anchor
A picture or mirror can establish the arrangement. Check its width against the furniture and install it safely. Leave enough space to use the sideboard without knocking objects over.
Vary the height
Try one taller object, a medium-height piece and a lower tray or bowl. A small group can feel connected through material or colour even when the shapes differ. Place heavier and fragile pieces securely away from an exposed edge.
Keep space in the composition
You do not need to fill the whole surface. An empty area makes the pieces easier to see and leaves practical space for daily use. Step back and remove anything that does not help the arrangement.
Use decorative pieces appropriately
A tray is not necessarily food-safe or water resistant. A vase may be intended only for dry stems. Confirm the intended use and protect the furniture surface from moisture, scratches and stains.