It's very easy to just write 'and add bias binding' to a tutorial. To someone who knows what bias binding is this is a simple instruction, but to someone who is new to bias binding it can be kind of nerve wracking.
I remember when I first started sewing I hated bias binding, okay I still hate bias binding, but I'm not scared of it anymore.
You can buy bias binding in a variety of colours from any craft/fabric store. Or you can make your own (this is a much more cost effective approach)
What is it made ouT of?
But, in theory, it can be made out of any kind of fabric. I just made some out of burlap.
On The Bias
The diagonal weave on a 45 degree angle from the selvage.
Cutting along the bias gives the binding more stretch and flexibility than the crosswise or lengthwise grains would have.
Making your own
They're available in a variety of sizes, the bigger they are the wider your binding will be, the smaller they are the thinner your binding will be.
Your bias ribbon maker will tell you how wide to cut the strips.
I remember in school we had a bunch of different ribbon makers. It's a real pain having to painstakingly measure out the width of each and every strip of fabric. So my teacher cut out width templates to go with each ribbon maker. They were long strips of card cut to the right width that you just had to trace onto your fabric.
My ribbon maker fits a strip of fabric that's roughly as wide as a standard plastic ruler, so I just use that as a template.
Sewing bias binding
Ideally you'll need to pull it fairly taught. You want it to just cover the stitching that's visible on the other side of your fabric. Then sew it again, fairly close to the edge on the binding.
Cornering with bias binding
Keep sewing all the way down to the end, until you're a few stitches away from the end (maybe about half a centimetre away) Then you stop. Back stitch. And remove your project from the machine.