How to put a chicken down?

I posted about a month or so ago about my limping chicken. I have been keeping her in a crate and trying to exercise her legs. ( once I learned it wasn't sprained or broke) I moved her into a larger crate hoping to encourage her to walk again and build strength in her legs. She has not improved. I added vitamins to her water as well with no improvement. She has since started to loose alot of weight despite the fact that i still feed her twice a day and include occasional treats like strawberries or grapes. Im afraid its time to put her down. There is nothing more i know to do. Any suggested painless methods? Any over the counter meds i could crush up for her to just send her to sleep? Or is a gun or knife the best solution? Please help, this is my first experience With this.
Thank you