How to bind off loosely
A lot of people have trouble binding off shawls and toe up socks. Here are a few different ways to make this simple for you. Keep in mind that you need this stretchy so it needs to be looser then your normal knitting.
1) I have long said bind off so loose it feels wrong. If it feels wrong you are doing it right. This is true, you will be making large loops that are extremely loose. Work the standard bind off and hold the loops on the RHN as you progress so that they don't tighten. After you have bound of about an inch, pull on the edge you have already bound off and see if there's resistance or if you need to make them even looser.
2) K2, K2tog TBL, [K1, K2tog TBL on right needle] across.
3) YO from back to front, K1, slip YO over, YO, K1, 3 sts on RHN, Pass the two sts right over the left. Continue across.
4) Use a larger needle on the right side, for some this is all it takes.
5) K2tog TBL, sl st from RHN back to the LHN, continue across.
Some people like one technique over the other, everyone has their favorite. I'm sure there are many more,,, if you have a favorite that's not on here put it into the comments below to include it.
RHN Right Hand Needle K Knit K2tog Knit 2 together
LHN Left Hand Needle YO Yarn Over St Stitch(es)
TBL Through the Back Loop SL ST slip stitch