When dressing dolls it is often better to have a loose neckband, so the garment fits over the dolls head. If the neckband is ribbed, then the easiest way to do this is to cast [bind] off in the same rib pattern.
So if you are working k 1, p 1 rib proceed as follows
k 1, p 1:
and then pass the knit stitch back over the purl stitch to cast off the stitch:
and pass the purl stitch which was already on the right hand needle back over the stitch just knitted:
and pass the previous knit stitch back over the stitch just purled:
Continue working in this way until all the stitches are cast [bound] off.
The top of the cast off [bound off] edge will look as if the stitches go from one side to the other when looking down on it, the rib pattern pulls the stitches up close round the neck when a doll is wearing the garment and allows a lot more stretch than if all the stitches were cast off in the same way, such as by knitting them.