Good for you on making this important choice and facing your fears. My memory of the injection itself is like others a very brief (literally micro-second) sharp. scratch or pinch feeling. If you concentrate on breathing deeply and also wiggling your toes wildly at the same time, you will be through it in no time. You could also listen to a song on an iPod headphones or something if distraction works for you, but honestly it will be over in seconds.
Vaccines in general can be administered particularly quickly because unlike, say some medications or for blood tests, they are not injected into a vein which requires real precision, but into either the muscle or the fat, so a much larger surface and much less precision needed.
As a general rule and particularly in hot summer weather, I feel injections less if I am well hydrated so drink plenty of water and maybe have a bar or something to hand in case you phobia leaves you feeling a bit off afterwards. But no need for any particular special preparations.
For after effects, I recall the evening after I had that vaccine, my arm muscle felt a bit stiff and sore if I raised it up high, a distant dull ache as if I had been bruised or punched. I don't have needle phobia but it definitely wasn't the sort of pain I imagine a needle phobic would fear, just a bit of a dull bruise-like ache, only in certain movements. It was gone by morning.
Seriously, congratulations on doing this important thing for your baby despite your fears. Maybe this will be the moment that cures your phobia - wouldn't that be a nice story to tell your child when he or she is older?