Q: The last direction in making the base says "Using 1/4 flat, miter the base." We're not sure what "miter" means.
A: To "mitre or miter" a base is basically the same idea as twining the base. When you twine you use round reed, when you mitre you use flat reed.. It is merely a way of locking the spokes into place before turning up the sides and it also adds another 1/2" to the width and length measurements..
Simply weave over/under the spokes, when you get to a corner, you will "mitre" the reed, simply fold it over itself so the opposite side is up. Continue weaving over/under. Make the fold or right angle at each corner. You must select a "nice" piece of reed as two sides of the basket will have the right side out, two sides will have the wrong side out. However, since it is on the base, it is not that important.
This is also called a "locking row".