How to put my pet rat out of her misery?

I have never hurt an animal and never wanted to, but my pet female rat who is blind is dying. I think the other rat in the cage was stealing her food, even after I gave her the food myself, and she starved to death. She is literally only breathing, and kind of jerking her body occasionally, but other than that there is no movement and her mouth and eyes are open. We can't take her to the vet. It's too late. :( Is there a way to put an animal this small out of misery, as humanely as possible? I really don't want to hit her or anything. Is it best to just let her suffer? It's a really hard situation for me, as I love my pet rats, and never wished this for happen. I just want her to pass on and leave the pain behind.