Need help, I'm freaking out!

I bought it today. The website said leafless cut, so I prepared a clear plastic box (not too tall) and perlite, RO water, rootgrow formula. However, I've never had a cut this size. I only had leafless cuts. Oh and this is my first philodendron. What should I do to keep it from dying? For now I've washed it with insecticide (a mixture of homemade, soap, etc), now it's dripping in the shower. And I'm panicking. Help me!