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I think all you would need is to start from the top with an I-cord. Probably three stitches, go for about 2 inches so you can tie it in a knot for the nip. Make your increases until your hat is able to slide onto a grapefruit (about a 12 inch circumference, then knit for 2 inches and rib for an additional inch.

In other words lets say you have a yarn of your choice and size 5 needles. Your gauge is 5sts to the inch. You know you need 60 stitches to make a 12 inch circumference hat.

Cast on 3 stitches on a single double point needle
Work 2 inches or however much you need to tie the knot of I-cord.
Knit one, make one, knit one, make one, knit one, make one across needle (use whatever increase you prefer and is appealing to your eye), and split your now six stitches across three double point needles placing a beginning of round marker, you are now beginning to work in the round.
Repeat last round across all three needles (12 stitches)
*Knit 2, make one* repeat across all three needles (18 stitches)
*Knit 3, make one* repeat across all three needles (24 stitches)
*Knit 4, make one* repeat across all three needles (30 stitches)
Knit one round
*Knit 5, make one* repeat across all three needles (36 stitches)
Knit one round
*Knit 6, make one* repeat across all three needles (42 stitches)
Knit one round
*Knit 7, make one* repeat across all three needles (48 stitches)
Knit one round
*Knit 8, make one* repeat across all three needles (54 stitches)
Knit one round
*Knit 9, make one* repeat across all three needles (60 stitches)
You now have your 60 stitches! Did you see how all those increases fell? You had three needles and all your increases fell in the middle and at the end of each needle giving you a total of 6 points of increase around your hat working towards 60 in a multitude of 6 making it an easy count to keep track of and a nice rounded hat.
Now, knit all rounds until the measurement from your last increase to your working edge is 2 inches. Then work in a 2 by 2 (knit 2, purl 2) ribbing for an additional inch leaving you from your last increase toy the edge of your work with 3 inches.

Cast off using a method such as the surprisingly stretchy cast off so you don't leave funny creases in baby heads. And you now have a boob hat. Just remember to throw in your color change where you think it should fall for a nipple of your own size taste... I'm not saying anything, there's no judgement here. So, start with the nipple color, and move into your breast color.

Hope that made sense to you.