I've been giving all shots to my horses(and to my large dogs, and my cat, when I had one-except for Rabies, which can't be ordered in NM)for MANY years; it is really quite simple, but you do need to stay focused and follow some basic proper procedures. I order from whatever catalog company has the best overall 'deal'; since I order BEFORE the weather warms up, I don't need to use special shipping, just the standard ice packs; I make sure I will be home on delivery day, so the vaccines can go straight into the fridge upon delivery. This year, I got my 6 way from Jeffers, and my West Nile from a vet supply house in my own state; it came in one day, and the ice packs were still nearly frozen solid! I give a 6 way, and West Nile(Recombitek, though I may well try the newest and supposedly, best WN after it's been on the market for a bit longer.)I do NOT give them at the same time, but wait about 10 days-2 weeks between times of administration, as I believe that ONE such 'stressful to their system' procedure at a time is enough(I do not deworm at the same time, etc., either.) I buy 3 cc. syringes and 1" X 21 or 22 ga. needles in boxes of 100(am still on boxes I bought some years ago, since I have less horses!--on my 'big' horse, I use a 1 1/2" X 20 ga., the size that comes w/ all individual dose vaccines.)
I give in the buttocks, unless I absolutely HAVE to use the neck(as it IS easier to do in the neck; but worse if soreness or an abcess develops-in the buttocks, gravity will be your friend if an abcess should form-I've had only one, a minor one in the buttock that was already 'burst', drained, and starting to heal before I even saw it, as it was hidden by a luxuriant amount of TAIL....develop in over 30 years of giving my own shots to all of my horses. "Shake"(roll it rapidly between your palms, actually) well; if using a multidose vial, carefully open the needle, place on syringe, draw a bit of air into the syringe, then insert into the vial, and insert the air, DON'T pull back on the plunger until you have upended the vial, so that the tip of the needle is IMMERSED in the liquid, then gently pull the required amount into the syringe. Turn the vial right side up again, so that the needle tip is again OUT of the liquid, before carefully and slowing withdrawing the needle and syringe. Have freshly-washed hands when you start this process, keep your fingers OFF the needle itself, the syringe tip, the top of the vaccine vial. I immediate pop the needle cover back onto the needle after filling the syringe, and do not remove it again until I am ready to place the needle into the horse. I ALWAYS work alone; I tie the horse up fairly short and where I can(with a mini, at least!) push them over against a safe secure fence if need be. I do NOT do the 'thumps' -the horses got wise to that REAL quick--I just eyeball carefully where I want to place the needle, aim, then 'pop'(don't "WHAM")it in, to the hub, then reattach the syringe, pull back to be sure I'm not in a vein/artery. If not, then push the vaccine in, firmly but not TOO fast, completely, then a quick withdrawal of the needle and massage the area for a moment or two. If you HAVE to give in the neck, be SURE you are in the relatively SMALL , triangular, 'target area'(your vet should show you). Pinch up the skin right next to where you intend to place the needle, shake the skin a bit--you can leave the syringe attached to the needle for this kind of shot, just 'SLIDE' it in next to where you have pinched the skin. Then draw back the needle; if no blood is seen, complete as described above. ABSOLUTELY, use ONLY a clean needle( I always take extras out with me- I have had horses jump and shoot the needle out across the pen into the dirt(there is NO way to clean it so it's safe to use then!), or bend it so it wasn't usable, etc.-but these things don't happen often. The main thing, I think, is to be focused on what you are doing, and be calm and deliberate in your actions.
It IS a VERY good idea to keep a vial of epinepherine on hand if you give your own shots; it could save the day if there is an allergic reaction. I always 'hang around' for 15-30 min. after giving shots, because if there are to be allergic reactions, they will almost certainly occur within that period of time.
Good luck! Practice as suggested, stay calm and focused, have a plan in place should anything not go as planned.
Margo