Start with a cardboard box. I mean, you could buy a jumbo paper mache letter or number from the craft store, but why bother when we all know you've got an Amazon box or 2 (or 3 or 4) lying around.
I took a pencil and free-handed my numbers. Just kept redoing it until I got them right. It really doesn't matter how it looks since it will all be covered up with flowers.