I start by making the piece larger than it will end up being. For the 8.5 x 11 pieces, I made them approximately 9 x 12. After the piece is quilted I use a ruler and permanent marker to mark the final size on the top of the quilt. Then I lay a piece of perle cotton on the marked line and zigzag over it. By doing this before you cut the edges off, you can get it nice and smooth, with neat corners. I use a narrow zigzag. On my Janome it is 1.5. Don't start and end at a corner. The start and stop are less noticeable along a side.After stitching the perle cotton down, carefully trim the excess fabric off, as close to the stitched perle cotton as you can get without cutting the stitching.
Now you are going to zigzag over the perle cotton and the edge again, but before you do that, take a needle threaded with a heavy-weight thread. I use buttonhole thread. Just take one stitch through each corner of your piece and cut the thread off, leaving two long tails, about 3" long.
You know how when you zigzag around the edge of something the corners are a mess and sometimes they get caught down in the hole or hang up on the foot? Well, those thread tails you just made are there to alleviate those problems! As you approach a corner, grab the the thread tails and use them to guide the stitching up to the corner. Stop, with your needle down at the corner, pivot and use the thread tails to pull the corner gently toward the back, under the foot, while you smoothly stitch into the next straight side.When you are finished, you can just pull the thread tails out and discard. And there's my finished piece with a nice, narrow little zigzagged edge. Isn't that neat?
P.S. A couple of people have asked what size perle cotton I am using. Does it come in different sizes?! The label has a 5 on it—I guess that's the size. It is not nearly as fat as yarn. The finished edge is only about 1/16th of an inch wide. I'm sure you could make it wider if you wanted to use something fatter than the perle cotton.
P.P.S. I am not using a satin stitch on this. The length of the stitch is just the default stitch length. I think the tight satin stitch is often another thing that contributes to bad corners and hangups. The perle cotton fills in under the zigzag stitch so that it covers it pretty well and a satin stitch isn't really needed. I must admit, also, that I have been known to take a permanent pen and touch up spots where the light fabric shows through! If you want more coverage you can stitch around the edge one or two more times.