Short answer: Measure dough accurately, knead until smooth, proof until appropriately expanded, avoid overproofing, and manage condensation during steaming. Let pressure and temperature settle briefly before opening the lid.
Sudden cooling can wrinkle buns, while water dripping from the lid damages the surface. Wrap the lid if the steamer design makes that safe, or angle it so condensation does not fall on the dough. Store cooled buns airtight and re-steam gently rather than microwaving too long.