Ladyhawke1 is absolutely correct. I hate to throw more nerves your way, but injections are not easy in any animal. I worked in 2 clinics, please get some pro advice or help. And if your vet said do it, and didn't show you how, inject the vet!
If the penicillin came from the feed store, think twice! Check the expiration date, look to see if it's injectable or not. As a past vet assn't, I still prefer tetracycline in the water to injectable anything.
Either way, if you have a hen on antibiotics, don't eat the eggs for at least 4-6 weeks- throw them away.
What is the problem, anyway? Can any of us help?