400 is highly concentrated. I had 350 and that sht hurt.
1) Warm it up by running hot water over it for a few minutes. Don't get any inside the needle obviously. You don't want to inject tap water. Some people use a blow dryer to heat it up.
2) Inject slowly. Use a 25g needle and push it in while keeping it steady over the course of one minute.
3) Use a heating pad for 5 minutes after injecting and rub the injection site.
It will get better. I remember the first shot; them Parkinson shakes lol. If there's any red spot, swelling, fever, or heat radiating from the shot then circle the redness. If redness spreads then getcho ass to a doctor and get some amoxicillin.