Lunaki, I found this site. Scroll down and you'll see a feeding tube with syringe attached and some instructions. I got a feeding tube from my vet. It is a 12 French in size. You can use other tube material if you can't get the "real thing."
I would add to the instructions on the website. First, hold the tube on the outside of the hen's body so that you can place a mark with a sharpie or something permanent to show you when you have inserted it far enough. I found that to be helpful. Just put the tip toward the bottom of the crop you can see on the outside and then make your mark where the tube is at the beak.
To insert, hold the hen's head in your left hand (if you're right handed) so that the top of her head is touching your palm. You are just coming down on her head with your hand. Then, as you wrap your fingers around her head to hold her, you can use your index finger to pull her beak upward. Your thumb and other fingers under her neck will help hold her. She will squirm a bit at first but after you've done this a few times, they don't seem to object. Also, put a little vegetable oil on the outside of the tubing so it will slide easily.
Look for the large opening where food goes into the crop. You will see her tongue and it's at the back of the tongue. The air passage is under the tongue so you must be sure to let that tube slide down the back of the tongue.
It sounds difficult but it isn't. Once you get the tube in there, you must keep holding her head or she'll shake her head and it will come out. Once you've injected the warm water, slowly withdraw the tube.
If this doesn't make sense, please post questions. You can do this!