An intrermuscular injection is going to be a lot more painful for the animal. Therefore, it is necessary that you have a plan for adequate restraint. The milk stand is the perfect answer. Otherwise, it might be best to get someone to help you hold your victim.
An IM shot is usually given to a goat in the muscle of the upper rear leg, better known as the butt. You can use the large muscles of the neck or shoulder if you're somewhat experienced; but best to start with the rear end. If your victim isn't used to having shots, you can give a couple of smart slaps to the target area which helps to reduce the startle of the needle. Then firmly push in the needle (all the way if using a 1" needle). Pull back the plunger to see if you've located a blood vessel. If you see blood entering the syringe let go of the plunger (don't push the blood back in; leave it in the syringe), back out and try a different site. Keep pulling back on the plunger at each site until you find one that doesn't yield blood. Slowly push in on the plunger until the contents have been injected. (Generally, you don't want to inject more than 10ml at any one site. When done, swiftly remove the needle making sure that you're not pushing in on the plunger while you're withdrawing.
Watch the animal for about one-half hour to make sure that it's not going to have some sort of reaction (See page in SQ shots). Never give long-acting (aka 24 hour) penicillin by the IM route. The results could easily be fatal.
