Spray with sizing and iron. I soak mine with sizing. I like them stiff because they are easier to work with. Turn it over and spray the other side. Set aside to let it dry. I like to let it dry overnight for the stiffest results. But you can work with it if you iron it as dry as possible. You just have to be more careful to maintain the circular shape after you remove the cardboard circle. A hair dryer might dry it quicker also. Just an idea.
Tip: I thread several needles at a time so you have what you need at hand.
Tip: If you don’t want to get sizing all over your ironing board ( it can get sticky) you might protect your ironing board with a thin cotton tea towel before you spray. You still need a flat surface to iron on so don’t use a fluffy towel or you will not be able to get it flat enough