One set of instructions; two gauges, with two different yarn / needle combinations, give us the two different hat sizes - newborn & infant. You’ll only need one needle size per hat. (Although, if you’d like to add the optional ties, I recommend 2 dpns in a size or two smaller than your hat needle.)

My recommendations:

Use sport weight yarn with either a US #2 / 2.75mm or US #3 / 3.25mm needle for the 0-3 month size.

Use DK weight yarn with either a US #5 / 3.75mm or US #6 / 4.0mm for the 3-6 month size.

The navy / white hat was knit using Dale Terne, which has been discontinued. The blue / yellow hat was knit using Dale Lerke, which is no longer available in North America. Several excellent yarn substitutes have been listed, but there are plenty more to chose from. Go with sport weight for the newborn size, DK weight for the larger size.