Measure circumference of head just above top of ears. Wrap tape gently around head making certain tape lies midway on the occipital bone (that little bump in the middle of the back of your head). *Don't pull the tape too tightly or your hat will fit that tightly.
The sizes above are average head sizes for ages, but your baby's head may be larger or smaller. Measure your baby's head first before ordering. Hats are sized large. For example a 0-3 month hat should fit a large headed 3 month old well.