You'll need a large, flat surface to pin it to. I used our bed, but you could also use a carpeted floor or those interlocking foam squares. Starting from one end, take one of the longer wires and thread it though the stitches along one long edge. Make sure there are at least a few inches of wire left at each end. It will probably take more than one wire to do each long edge (this scarf took 3 for each side). Do the same for the other long edge.
Now take one of the short wires and thread it through the points of a short end. Do the same for the other side. If the short ends are straight too, thread it through the stitches like you did for the long edges.
Now get your pins out. Splurge for some heavy-duty T-pins. They are long enough that you can pin into the bed through a blanket, and strong enough they won't bend. Spread one end of the scarf out to the width you want and pin that end wire to the bed. Do the same for the other end, then pin the side wires every so often. There needs to be enough pins to keep the sides parallel, but you don't need to go crazy. I wound up doing every 8-12 inches or so.
If it's a nice day, open the windows so it will dry faster. If you have pets, you may want to close the door to that room for a few hours. Otherwise, just wait until it is completely dry. This one took about 4 hours on a 45 degree day with the windows closed. Once it's dry, remove the pins, then remove the wires. Voila!