And this is how I go about prepping my rug to be whipped with either wool strips or yarn. At first I tried to roll and whip in one process but found my rolled up edge got distorted and my whipping lumpy. So while it seems like double work, I find it much easier to roll the backing forward and use T-pins to secure small sections at a time and then whip that with cotton crochet yarn.
Notice that I have rounded my edges vs. sharp corners. This makes it easier for me to roll those sections and there is less bulk at the corners when I roll forward. I use the cotton crochet yarn because it is readily accessible at Wal-Mart and is inexpensive.
I start in the middle of a side rather than starting at the corner; I find that easiest for me. I take my T-pins and pin a section and then I whip stitch that together, pin a little further along the edge and then whip stitch that together.
This is to be continued tomorrow. Hope this has been helpful.