Arrange sweet potatoes on a steamer rack in a large pan over 1-inch of water. Cover and bring to a boil. Reduce heat. Steam until tender; about 12 minutes. Pulse in a food processor or pass through a food mill fitted with a medium disk. Add formula, breast milk, or boiled and cooled water as needed. Fill 1/4 cup plastic containers or ice cube trays with puree. Use same method for new or baking potatoes and any winter squash.