People have more trouble with those directions. I wish they would clarify what they mean. I did another one of these pot holders a while back to remind myself how it works. But I worked mine over 31 stitches (I still have it sitting here on the needle).
Do you belong to Ravelry? Do you know about it? Anyway there is a group on that site devoted to double knitting and one of the guys on there is a real expert. He has a few free videos he has links to. You might be able to look his videos up on YouTube, his name is fallingblox (I think).
On this last potholder I did I used his idea of how to deal with the edges, but you need at least 2 stitches on each side in the outside border color to make it work so I used an extra stitch. 31 instead of what they said
On the predominately red side I have 2 stitches of red then the white border they have (1 to three white sts) then past all 3 of the border sts I have 21 red stitches then the border again (from 1 to 3 stitches), then 2 stitches of red.
I believe that the way it works is when you get to the last 2 stitch loops on the needle (one of each color) that you slip the first one purl wise with both yarns in back, then with both yarns in front slip the last stitch purl wise. Then you work the first stitches of the next row the same way you work all the rest.
I say, I believe that is how it works. I just tried again on a tiny sample and I think that is right.