With a waste yarn in contrasting color, cast-on half the number of stitches you need using the chained cast-on. If your pattern calls for an odd number of stitches, add one to that number and then divide by two. Switch to your main yarn. Purl one row, knit one row, twice (total of 4 rows):
Now you're going to insert the right-hand needle into the purl bump from the first row. It's intermixed into the waste yarn. Here's where your contrasting yarn colors are key.
Insert RH needle from top to bottom (can also be described as from the front of the stitch to the back):
Place the stitch on the LH needle from bottom to top (back of the stitch to the front). Knit the stitch:
Repeat to the last stitch. If you wanted an odd number of stitches, purl the last stitch and you're done. If you wanted an even number of stitches, knit the last stitch, as well as the last purl bump. It will be a selvage edge, so it may be pulled fairly tight, and you'll have to work to get your needle in there. Here's what we've got so far:
Do a few rows of K1, P1 ribbing, then you can start to carefully take out the waste yarn. If you undo the last stitch cast-on with the waste yarn, you should be able to pull the rest out by tugging on the end (which is what makes the chained cast-on a sweet provisional cast-on). Check it out:
You'll notice that part of the waste yarn ends up in the tube you created with the cast-on. You can carefully pull it out. If you wanted an elastic band in your garment for extra pull on the edge, this is the ideal spot. You can thread it in post-mortem, or you can use the elastic as your 'waste yarn' if it is an appropriate shape.
And here's the finished product!
I *heart* this so much. It's gorgeous and definitely worth the extra time spent.