How do I avoid my blanket binding ruffling up?

If you're applying the binding to something stretchy, it could be you need to use a zigzag stitch. If you're not using stretchy materials, be sure to pin the binding in place. If that isn't it either, I would wonder if the tension is set properly. Try stitching on scrap material - are the stitches nice and flat without pulling? Next, try applying a small piece of binding to scrap fabric; if it's not ruffled, then maybe you are using too much pressure when applying your blanket binding. There are a lot of variables, so you may have to test out a few different solutions to find the right one.