I've done a ton of these, and if you forget to get it started before trimming here's an easy way to do it...
The paper backing tears easily; the transfer paper doesn't. I know it's scary at first, but you can make a tiny gentle tear to the paper backing while it is still attached to the transfer paper and the transfer paper will just stretch while the backing tears. This makes it SO easy to separate! Test it out on the untrimmed portion to give you confidence. I still usually start it that way on the untrimmed partand then cut a slit in the backing that extends into the area I'm about to trim, then I trim it and peel it. However, when you forget, don't be afraid to tear the backing--it really works. And saves oh so much time and frustration!