You do not need to be precise with this, as you can always trim it at the end. You can also choose to make your strips longer or shorter than mine, depending on where you plan to hang them and the look you like.
If you are using longer pieces of fabric scrap, you simply fold it in half, make a loop, slide the loop under the twine, and feed the ends through it. Then pull tight to secure.
If you are using half lengths, tie a regular knot onto the twine at one end of the fabric. Try to tuck the short end underneath and out of sight, or go back and trim it off when you are finished.
I gave my bunting a bit of a trim when I was finished. as I was using a mixture of longer and shorter strips. You can decide how precise you want to be about that!