To begin, cut dahlia petals in 3 sizes from a sheet of felt. You'll need 6 large petals, 6 medium, and 6 small.
You can find a template of these petals on my blog. See https://aboxoftwine.com/how-to-make-felt-dahlia-flowers/. (NOTE - you'll need to subscribe to my email list to gain access to this template & other freebies)