I am going to make another hat. I have some nice handspun wool that has been sitting in the stash for about 8 years, and I need to make a Christmas present. The yarn is about a sport weight and I'm using size 5 needles.
First I guesstimated that 18 sts would give me about 4". (As it turns out, it's working up to about 4 1/2", but that's close enough).
I'm working plain old garter stitch, but I am slipping the first stich of every row knitwise and purling the last stitch of every row. One of those edges is going to show, so I want it to look smooth, and I will be picking up stitches along the other, which will be easier with a chain selvedge.
Now I will just keep knitting until the piece measures about 18". Why 18"? Because the ever so handy Standard Body Measurements/Sizing chart gives that as the head circumference for a child, and the intended victim for this one is, indeed, a child. I am not worrying about adding ease, because that garter is going to be stretchy. (On the other hand, this particular child has a largish head - maybe I'll go to 19", which is what the Yarn Harlot lists as average for a 5-10 year old.)
La, la, la; knit, knit, knit.
OK at, about 18 3/4" I stopped knitting and joined the two short ends together. I did this with an odd "grafting the live stitches to the cast on" move. I could have just bound off and sewn the ends together, but this would have made a thicker seam. I could have had an invisible graft if I had only thought to use a provisional cast on, but I didn't, and this is reasonably unobtrusive.
Using a short circular needle, I picked up one stitch from under each slipped edge stitch. I did this with the lumpier side of the seam towards the outside, because the garter cuff will be folded up when the hat is done.
Now I'm just going to knit round for a while.
La, la, la; round and round.
(edited to add: to learn how to work the top, jump to The Rest of the Story)