This can really be made any length and size you want, just make a cardboard template for the bunting flags, and cut along the edge with pinking shears to stop fraying. Remember the top of the flags will be covered by the ribbon.
Take a piece of ribbon the length you want, fold and half and iron along the length to make it a bit easier. Then measure along the length so the flags are equal lengths apart, leave a bit extra at the end so you can make loops to hang it with. Pin in place and sew along with a sewing machine, or by hand! turn the edges in to make loops and sew on the back side.