You'd need to inoculate before pollination/during the pollination period. You'd save the spores and put them on the silks (maybe in a dropper mixed with water) when they start showing. Probably next year at this point. Some sources say freezing is bad for saving the spores. Some say it's fine. Keeping them pretty dry is probably going to be the most important part.

I'd probably let them dry in a cool garage/cellar/cupboard in an open jar and then cap that until the right time next year.

If your corn has already started forming kernels and filling the husks, it's too late for new fungus to take hold.