Step 8: Attach the binding: mitered corner
This is the most fiddly part of the whole process. When done right, the corner looks very neat and tidy.
Pinch the edge of the blanket and open up the satin. Bend the satin so that the corner of the satin and the corner of the blanket are at the same place, then close the satin so that there are satin folds coming out at 45 degree angles from the corner. Pin.
Now sandwich the rest of the blanket edge between the satin and pin to secure.
Since the satin is slippery, the folding can get a little funky. I usually pin the back of the blanket then the front. Then put pins all the way through and remove the first ones. The trick is to get the 45 degree folds to line up on top of each other. When you sew, you want the zig-zag to straddle the two edges, so the folds you have to be symmetric to within your stitch width.
Now that you have your folds pinned, use zz1 to secure the folds near the corner (back and forth about 4 stitches each), then switch to zz2 to finish the 45 degree folds, turn the corner, and sew the rest of the blanket side. Again, adjust the pins if needed, just make sure that the middle satin fold lines up with the flannel edge.
Repeat this twice more. For the last corner, pin, but don't sew yet.